diff --git a/backend/db/migrate/20261008000000_create_tag_groups.rb b/backend/db/migrate/20261008000000_create_tag_groups.rb
new file mode 100644
index 0000000..f113502
--- /dev/null
+++ b/backend/db/migrate/20261008000000_create_tag_groups.rb
@@ -0,0 +1,149 @@
+class CreateTagGroups < ActiveRecord::Migration[8.0]
+  def change
+    # 翻訳系は上流の Translation リソースが未完のため保留
+
+    create_table :tag_groups do |t|
+      t.integer :version_no, null: false
+      t.string :name, limit: 255, null: false
+      t.text :description
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.references :updated_by_user, foreign_key: { to_table: :users }
+      t.timestamps
+
+      t.check_constraint 'version_no > 0', name: 'chk_tag_groups_version_no_positive'
+      t.index :name, unique: true
+    end
+
+    create_table :tag_group_tags, primary_key: [:tag_group_id, :tag_id] do |t|
+      t.references :tag_group, null: false, foreign_key: true, index: false
+      t.references :tag, null: false, foreign_key: true
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.datetime :created_at, null: false
+    end
+
+    create_table :tag_group_groups, primary_key: [:tag_group_id, :child_tag_group_id] do |t|
+      t.references :tag_group, null: false, foreign_key: true, index: false
+      t.references :child_tag_group,
+                   null: false, foreign_key: { to_table: :tag_groups }
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.datetime :created_at, null: false
+
+      t.check_constraint 'tag_group_id <> child_tag_group_id',
+                         name: 'chk_tag_group_groups_tag_group_id_not_child_tag_group_id'
+    end
+
+    create_table :tag_group_placements, primary_key: [:tag_group_id, :position] do |t|
+      t.references :tag_group, null: false, foreign_key: true, index: false
+      t.integer :position, null: false
+      t.integer :parent_position
+      t.string :section_name, limit: 255
+      t.text :description
+      t.references :tag
+      t.references :child_tag_group
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.references :updated_by_user, foreign_key: { to_table: :users }
+      t.timestamps
+
+      t.foreign_key :tag_group_placements,
+                    column: [:tag_group_id, :parent_position],
+                    primary_key: [:tag_group_id, :position]
+
+      t.foreign_key :tag_group_tags,
+                    column: [:tag_group_id, :tag_id],
+                    primary_key: [:tag_group_id, :tag_id]
+
+      t.foreign_key :tag_group_groups,
+                    column: [:tag_group_id, :child_tag_group_id],
+                    primary_key: [:tag_group_id, :child_tag_group_id]
+
+      t.check_constraint 'position > parent_position',
+                         name: 'chk_tag_group_placements_position_gt_parent_position'
+
+      t.check_constraint ('((section_name IS NOT NULL AND tag_id IS NULL AND child_tag_group_id IS NULL) OR ' +
+                          '(section_name IS NULL AND tag_id IS NOT NULL AND child_tag_group_id IS NULL) OR ' +
+                          '(section_name IS NULL AND tag_id IS NULL AND child_tag_group_id IS NOT NULL)) AND ' +
+                          '(section_name IS NOT NULL OR description IS NULL)'),
+                         name: 'chk_tag_group_placements_references_exclusive'
+    end
+
+    create_table :tag_group_versions do |t|
+      t.references :tag_group, null: false
+      t.integer :version_no, null: false
+      t.string :event_type, null: false
+      t.string :name, limit: 255, null: false
+      t.text :description
+      t.json :members, null: false
+      t.json :placements, null: false
+      t.references :created_by_user
+      t.datetime :created_at, null: false
+
+      t.check_constraint 'version_no > 0',
+                         name: 'chk_tag_group_versions_version_no_positive'
+      t.index [:tag_group_id, :version_no], unique: true
+
+      add_members_constraint!(t)
+      add_placements_constraint!(t)
+    end
+  end
+
+  private
+
+  def add_members_constraint! t
+    schema = { type: 'array',
+               uniqueItems: true,
+               items: member_schema }
+
+    quoted_schema = connection.quote(JSON.generate(schema))
+
+    t.check_constraint "JSON_SCHEMA_VALID(#{ quoted_schema }, members)", 
+                       name: 'chk_tag_group_versions_members_schema'
+  end
+
+  def member_schema
+    { oneOf: [tag_member_schema, group_member_schema] }
+  end
+
+  def tag_member_schema
+    { type: 'object',
+      required: ['tag_id', 'version_no'],
+      properties: { tag_id: { type: 'integer', minimum: 1 },
+                    version_no: { type: 'integer', minimum: 1 } },
+      additionalProperties: false }
+  end
+
+  def group_member_schema
+    { type: 'object',
+      required: ['tag_group_id', 'version_no'],
+      properties: { tag_group_id: { type: 'integer', minimum: 1 },
+                    version_no: { type: 'integer', minimum: 1 } },
+      additionalProperties: false }
+  end
+
+  def add_placements_constraint! t
+    schema = { type: 'array', items: placement_schema(5) }
+
+    quoted_schema = connection.quote(JSON.generate(schema))
+
+    t.check_constraint "JSON_SCHEMA_VALID(#{ quoted_schema }, placements)",
+                       name: 'chk_tag_group_versions_placements_schema'
+  end
+
+  def placement_schema rest
+    { type: 'object',
+      required: ['children'],
+      properties: { section_name: { type: 'string' },
+                    description: { type: ['string', 'null'] },
+                    tag_id: { type: 'integer', minimum: 1 },
+                    tag_group_id: { type: 'integer', minimum: 1 },
+                    children: if rest > 1
+                                { type: 'array', items: placement_schema(rest - 1) }
+                              else
+                                { type: 'array', maxItems: 0 }
+                              end },
+      oneOf: [{ required: ['section_name'] },
+              { required: ['tag_id'] },
+              { required: ['tag_group_id'] }],
+      dependencies: { description: ['section_name'] },
+      additionalProperties: false }
+  end
+end
diff --git a/backend/db/migrate/20261008010000_migrate_tag_groups_from_wiki_pages.rb b/backend/db/migrate/20261008010000_migrate_tag_groups_from_wiki_pages.rb
new file mode 100644
index 0000000..9e68d7e
--- /dev/null
+++ b/backend/db/migrate/20261008010000_migrate_tag_groups_from_wiki_pages.rb
@@ -0,0 +1,518 @@
+class MigrateTagGroupsFromWikiPages < ActiveRecord::Migration[8.0]
+  PREFIX = 'タググループ:'
+
+  def up
+    require 'json'
+    require 'time'
+    require 'kramdown'
+    require 'kramdown-parser-gfm'
+
+    assert_empty_destination!
+
+    pages = connection.select_all(<<~SQL).to_a
+      SELECT
+        wp.id
+      , wp.created_user_id
+      , wp.updated_user_id
+      , wp.created_at
+      , wp.updated_at
+      , wp.discarded_at
+      , tn.name
+      FROM
+        wiki_pages wp
+        INNER JOIN
+          tag_names tn
+          ON
+            tn.id = wp.tag_name_id
+      WHERE
+        tn.language_code = 'ja'
+        AND tn.name LIKE #{ quote("#{ PREFIX }%") }
+      ORDER BY
+        wp.id
+    SQL
+
+    return if pages.empty?
+
+    pages.each do |page|
+      if page['discarded_at']
+        say "Wiki ##{ page.fetch('id') }: 破棄済みですが，Revision の移行を継続します．"
+      end
+    end
+
+    tag_ids = load_tag_ids
+    page_ids = pages.map { |page| page.fetch('id').to_i }
+
+    revisions = connection.select_all(<<~SQL).to_a
+      SELECT
+        wr.id
+      , wr.wiki_page_id
+      , wr.kind
+      , wr.redirect_page_id
+      , wr.created_user_id
+      , wr.created_at
+      FROM
+        wiki_revisions wr
+      WHERE
+        wr.wiki_page_id IN (#{ page_ids.join(', ') })
+      ORDER BY
+        wr.wiki_page_id
+      , wr.id
+    SQL
+
+    revisions_by_page = revisions.group_by { |row| row.fetch('wiki_page_id').to_i }
+
+    pages = pages.select do |page|
+      page_id = page.fetch('id').to_i
+      if revisions_by_page.fetch(page_id, []).empty?
+        # tag_groups.version_no > 0 のため，Revision 0 件の Wiki は作成しない．
+        say "Wiki ##{ page_id }: Revision が 0 件のため移行対象から除外します．"
+        false
+      else
+        true
+      end
+    end
+
+    return if pages.empty?
+
+    group_ids = pages.to_h { |page| [page.fetch('name'), page.fetch('id').to_i] }
+    revision_history = pages.to_h do |page|
+      page_id = page.fetch('id').to_i
+      history = revisions_by_page.fetch(page_id).each_with_index.map do |revision, index|
+        { 'id' => revision.fetch('id').to_i,
+          'version_no' => index + 1,
+          'created_at' => revision.fetch('created_at') }
+      end
+      [page_id, history]
+    end
+
+    tag_history = connection.select_all(<<~SQL).to_a.group_by { |row| row.fetch('tag_id').to_i }
+      SELECT
+        tag_id
+      , version_no
+      , created_at
+      FROM
+        tag_versions
+      ORDER BY
+        tag_id
+      , created_at
+      , version_no
+    SQL
+
+    prepared = { }
+    pages.each do |page|
+      page_id = page.fetch('id').to_i
+      history = revisions_by_page.fetch(page_id)
+
+      prepared[page_id] = history.each_with_index.map do |revision, index|
+        context = "Wiki ##{ page_id } / Revision ##{ revision.fetch('id') }"
+        body = revision_body(revision.fetch('id').to_i)
+
+        if revision.fetch('kind').to_i == 1
+          destination = revision['redirect_page_id']
+          description = "旧 Wiki のリダイレクト先: Wiki ##{ destination }"
+          placements = []
+          members = []
+          say "#{ context }: リダイレクトを説明文として移行します．"
+        else
+          begin
+            description, placements = parse_wiki(body, tag_ids, group_ids, context)
+            validate_tree!(placements, context)
+            members = build_members(placements, revision.fetch('created_at'),
+                                    tag_history, revision_history, context)
+          rescue StandardError => error
+            # 解析できない版も捨てず，当時の Markdown を説明文として保存する．
+            say "#{ context }: 構造化を保留し，本文を保存します (#{ error.message })．"
+            description = body
+            placements = []
+            members = []
+          end
+        end
+
+        { revision:, version_no: index + 1, description:, members:, placements:, body: }
+      end
+    end
+
+    # 自己包含は禁止されてゐるため，当該版は原文の保存を優先する．
+    prepared.each do |group_id, versions|
+      current = versions.last
+      next unless current.fetch(:members).any? { |member| member['tag_group_id'] == group_id }
+
+      say "Tag Group ##{ group_id }: 直接の自己参照を検出したため，現行構造化を保留します．"
+      current[:description] = current.fetch(:body)
+      current[:members] = []
+      current[:placements] = []
+    end
+
+    begin
+      validate_current_groups!(prepared)
+    rescue StandardError => error
+      say "Tag Group の包含関係に問題がありますが，移行を継続します (#{ error.message })．"
+    end
+
+    # 移行先の書込みはすべて検証の後．DDL は含めない．
+    connection.transaction do
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        current = prepared.fetch(page_id).last
+
+        insert_row('tag_groups', {
+            id: page_id,
+            # page.version_no ではなく，この Wiki に紐付く Revision の総数．
+            version_no: revisions_by_page.fetch(page_id).size,
+            name: page.fetch('name').delete_prefix(PREFIX),
+            description: current.fetch(:description),
+            created_by_user_id: page.fetch('created_user_id'),
+            updated_by_user_id: page.fetch('updated_user_id'),
+            created_at: page.fetch('created_at'),
+            updated_at: page.fetch('updated_at') })
+      end
+
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        name = page.fetch('name').delete_prefix(PREFIX)
+
+        prepared.fetch(page_id).each do |item|
+          revision = item.fetch(:revision)
+
+          insert_row('tag_group_versions', {
+              id: revision.fetch('id'),
+              tag_group_id: page_id,
+              version_no: item.fetch(:version_no),
+              event_type: item.fetch(:version_no) == 1 ? 'create' : 'update',
+              name:,
+              description: item.fetch(:description),
+              members: JSON.generate(item.fetch(:members)),
+              placements: JSON.generate(item.fetch(:placements)),
+              created_by_user_id: revision.fetch('created_user_id'),
+              created_at: revision.fetch('created_at') })
+        end
+      end
+
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        current = prepared.fetch(page_id).last
+        revision = current.fetch(:revision)
+
+        current.fetch(:members).each do |member|
+          if member.key?('tag_id')
+            insert_row('tag_group_tags', {
+                tag_group_id: page_id,
+                tag_id: member.fetch('tag_id'),
+                created_by_user_id: revision.fetch('created_user_id'),
+                created_at: revision.fetch('created_at') })
+          else
+            insert_row('tag_group_groups', {
+                tag_group_id: page_id,
+                child_tag_group_id: member.fetch('tag_group_id'),
+                created_by_user_id: revision.fetch('created_user_id'),
+                created_at: revision.fetch('created_at') })
+          end
+        end
+      end
+
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        current = prepared.fetch(page_id).last
+        revision = current.fetch(:revision)
+        position = 0
+
+        append = -> nodes, parent_position do
+          nodes.each do |node|
+            position += 1
+            own_position = position
+
+            insert_row('tag_group_placements', {
+                tag_group_id: page_id,
+                position: own_position,
+                parent_position:,
+                section_name: node['section_name'],
+                description: node['description'],
+                tag_id: node['tag_id'],
+                child_tag_group_id: node['tag_group_id'],
+                created_by_user_id: revision.fetch('created_user_id'),
+                updated_by_user_id: revision.fetch('created_user_id'),
+                created_at: revision.fetch('created_at'),
+                updated_at: revision.fetch('created_at') })
+
+            append.(node.fetch('children'), own_position)
+          end
+        end
+
+        append.(current.fetch(:placements), nil)
+      end
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+
+  private
+
+  def assert_empty_destination!
+    ['tag_groups', 'tag_group_versions', 'tag_group_tags',
+     'tag_group_groups', 'tag_group_placements'].each do |table|
+      count = connection.select_value("SELECT COUNT(*) FROM #{ table }").to_i
+      raise "#{ table } が空ではありません．" if count.positive?
+    end
+  end
+
+  # 解決は tag_names.name -> tag_names.tag_id のみ．
+  # tags.tag_name_id は読まない．
+  def load_tag_ids
+    rows = connection.select_all(<<~SQL).to_a
+      SELECT
+        name
+      , tag_id
+      FROM
+        tag_names
+      WHERE
+        language_code = 'ja'
+        AND tag_id IS NOT NULL
+    SQL
+
+    rows.each_with_object({ }) do |row, by_name|
+      name = row.fetch('name')
+      id = row.fetch('tag_id').to_i
+      by_name[name] ||= []
+      by_name[name] << id unless by_name[name].include?(id)
+    end
+  end
+
+  def revision_body revision_id
+    connection.select_rows(<<~SQL).flatten.join("\n")
+      SELECT
+        wl.body
+      FROM
+        wiki_revision_lines wrl
+        INNER JOIN wiki_lines wl ON wl.id = wrl.wiki_line_id
+      WHERE
+        wrl.wiki_revision_id = #{ Integer(revision_id) }
+      ORDER BY
+        wrl.position
+    SQL
+  end
+
+  def parse_wiki markdown, tag_ids, group_ids, context
+    root = Kramdown::Document.new(markdown, input: 'GFM').root
+    placements = []
+    description_parts = []
+    headings = []
+
+    root.children.each_with_index do |node, index|
+      case node.type
+      when :blank
+        next
+      when :header
+        level = node.options.fetch(:level)
+        name = inline_text(node).strip
+        raise "#{ context }: 名前のない見出しがあります．" if name.empty?
+
+        headings.pop while headings.any? && headings.last.first >= level
+        parent = headings.last&.last
+        section = { 'section_name' => name, 'description' => nil, 'children' => [] }
+        (parent ? parent.fetch('children') : placements) << section
+        headings << [level, section]
+      when :p
+        text = inline_markdown(node).strip
+        next if text.empty?
+
+        if headings.any?
+          heading = headings.last.last
+          heading['description'] = [heading['description'], text].compact.join("\n\n")
+        else
+          description_parts << text
+        end
+      when :ul, :ol
+        if headings.any? && see_also_heading?(headings.last.last.fetch('section_name'))
+          heading = headings.last.last
+          raw = original_block(markdown, root.children, index)
+          heading['description'] = [heading['description'], raw].compact.join("\n\n")
+        else
+          target = headings.any? ? headings.last.last.fetch('children') : placements
+          target.concat(node.children.map { |item| parse_list_item(item, tag_ids, group_ids, context) })
+        end
+      else
+        raise "#{ context }: 未対応の Markdown 構文 #{ node.type.inspect }"
+      end
+    end
+
+    [description_parts.empty? ? nil : description_parts.join("\n\n"), placements]
+  end
+
+  def see_also_heading? name
+    ['see also', '関連項目'].include?(name.strip.downcase.sub(/[:：]\z/, ''))
+  end
+
+  def original_block markdown, siblings, index
+    current = siblings.fetch(index)
+    start_line = current.options[:location]
+    raise 'Markdown block の位置が不明です．' unless start_line
+
+    following = siblings[(index + 1)..].find do |node|
+      node.options[:location] && node.options[:location] > start_line
+    end
+    end_line = following ? following.options.fetch(:location) - 1 : markdown.lines.length
+    markdown.lines[(start_line - 1)...end_line].join.strip
+  end
+
+  def parse_list_item item, tag_ids, group_ids, context
+    raise "#{ context }: リスト項目ではありません．" if item.type != :li
+
+    nested, own = item.children.partition { |child| [:ul, :ol].include?(child.type) }
+    own.reject! { |child| child.type == :blank }
+    unless own.all? { |child| [:p, :text, :a, :codespan].include?(child.type) }
+      raise "#{ context }: 未対応のリスト項目があります．"
+    end
+
+    label = own.map { |child| inline_text(child) }.join(' ').strip
+    raise "#{ context }: 空のリスト項目があります．" if label.empty?
+
+    node =
+        if group_ids.key?(label)
+          { 'tag_group_id' => group_ids.fetch(label) }
+        elsif tag_ids.key?(label)
+          ids = tag_ids.fetch(label)
+          unless ids.one?
+            raise "#{ context }: タグ名 #{ label.inspect } が複数の ID を指します：#{ ids.inspect }"
+          end
+          { 'tag_id' => ids.first }
+        elsif label.end_with?(':', '：')
+          name = label.sub(/[:：]\z/, '')
+          raise "#{ context }: 空の区分名があります．" if name.empty?
+          if see_also_heading?(name) && nested.any?
+            raise "#{ context }: リスト内の See also は説明文への保管処理が未実装です．"
+          end
+          { 'section_name' => name, 'description' => nil }
+        else
+          raise "#{ context }: 解決不能なリスト項目 #{ label.inspect }"
+        end
+
+    node['children'] = nested.flat_map do |list|
+      list.children.map { |child| parse_list_item(child, tag_ids, group_ids, context) }
+    end
+
+    node
+  end
+
+  def inline_text node
+    case node.type
+    when :text, :codespan
+      node.value.to_s
+    when :softbreak, :br
+      ' '
+    when :a, :p, :header, :em, :strong, :span, :li
+      node.children.map { |child| inline_text(child) }.join
+    else
+      raise "未対応のインライン構文 #{ node.type.inspect }"
+    end
+  end
+
+  def inline_markdown node
+    case node.type
+    when :text
+      node.value.to_s
+    when :codespan
+      "`#{ node.value }`"
+    when :softbreak
+      "\n"
+    when :br
+      "  \n"
+    when :p
+      node.children.map { |child| inline_markdown(child) }.join
+    when :a
+      href = node.attr.fetch('href')
+      "[#{ node.children.map { |child| inline_markdown(child) }.join }](#{ href })"
+    when :em
+      "*#{ node.children.map { |child| inline_markdown(child) }.join }*"
+    when :strong
+      "**#{ node.children.map { |child| inline_markdown(child) }.join }**"
+    else
+      raise "未対応のインライン構文 #{ node.type.inspect }"
+    end
+  end
+
+  def validate_tree! nodes, context, depth = 1
+    raise "#{ context }: 配置の深さが 5 を超えてゐます．" if depth > 5 && nodes.any?
+
+    nodes.each do |node|
+      name = node['section_name']
+      if name && name.length > 255
+        raise "#{ context }: 区分名が 255 文字を超えてゐます．"
+      end
+      validate_tree!(node.fetch('children'), context, depth + 1)
+    end
+  end
+
+  def build_members nodes, date, tag_history, group_history, context
+    ids_of_tags = []
+    ids_of_groups = []
+
+    visit = -> items do
+      items.each do |node|
+        ids_of_tags << node['tag_id'] if node.key?('tag_id')
+        ids_of_groups << node['tag_group_id'] if node.key?('tag_group_id')
+        visit.(node.fetch('children'))
+      end
+    end
+    visit.(nodes)
+
+    tags = ids_of_tags.uniq.map do |id|
+      { 'tag_id' => id,
+        'version_no' => version_at(tag_history[id], date, context, "Tag ##{ id }") }
+    end
+
+    groups = ids_of_groups.uniq.map do |id|
+      { 'tag_group_id' => id,
+        'version_no' => version_at(group_history[id], date, context, "TagGroup ##{ id }") }
+    end
+
+    tags + groups
+  end
+
+  def version_at history, date, context, target
+    raise "#{ context }: #{ target } の履歴が存在しません．" unless history&.any?
+
+    timestamp = Time.parse(date.to_s)
+    candidates = history.select do |row|
+      Time.parse(row.fetch('created_at').to_s) <= timestamp
+    end
+    if candidates.empty?
+      raise "#{ context }: #{ target } は #{ date } 時点で存在しません．"
+    end
+
+    candidates.max_by { |row|
+      [Time.parse(row.fetch('created_at').to_s), row.fetch('version_no').to_i]
+    }.fetch('version_no').to_i
+  end
+
+  def validate_current_groups! prepared
+    edges = { }
+    prepared.each do |group_id, versions|
+      edges[group_id] = versions.last.fetch(:members)
+                              .filter_map { |member| member['tag_group_id'] }
+    end
+
+    visiting = { }
+    visited = { }
+    visit = -> group_id do
+      raise "Tag Group の循環参照を検出しました：#{ group_id }" if visiting[group_id]
+      return if visited[group_id]
+
+      visiting[group_id] = true
+      edges.fetch(group_id, []).each { |child_id| visit.(child_id) }
+      visiting.delete(group_id)
+      visited[group_id] = true
+    end
+
+    edges.keys.each { |group_id| visit.(group_id) }
+  end
+
+  def quote(value) = connection.quote(value)
+
+  def insert_row table, values
+    columns = values.keys.join(', ')
+    payload = values.values.map { |value| quote(value) }.join(', ')
+    execute "INSERT INTO #{ table } (#{ columns }) VALUES (#{ payload })"
+  end
+end