diff --git a/backend/db/migrate/20260921040000_create_external_tags.rb b/backend/db/migrate/20260921040000_create_external_tags.rb
new file mode 100644
index 0000000..8afecaa
--- /dev/null
+++ b/backend/db/migrate/20260921040000_create_external_tags.rb
@@ -0,0 +1,54 @@
+class CreateExternalTags < ActiveRecord::Migration[8.0]
+  def up
+    create_table :external_tags do |t|
+      t.string :platform, limit: 16, null: false
+      t.string :name, limit: 255, null: false
+      t.integer :post_count, null: false, default: 0
+      t.datetime :created_at, null: false
+
+      t.index [:platform, :name], unique: true
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        external_tags(id, platform, name, post_count, created_at)
+      SELECT
+        t.id
+      , 'nico' AS platform
+      , SUBSTR(tn.name, 6) AS name
+      , t.post_count
+      , t.created_at
+      FROM
+        tags t
+        INNER JOIN
+          tag_names tn
+          ON
+            tn.id = t.tag_name_id
+            AND t.category = 'nico'
+    SQL
+
+    execute <<~SQL
+      INSERT INTO
+        external_tags(id, platform, name, post_count, created_at)
+      SELECT
+        ntv.tag_id
+      , 'nico' AS platform
+      , SUBSTR(ntv.name, 6) AS name
+      , 0 AS post_count
+      , ntv.created_at
+      FROM
+        nico_tag_versions ntv
+        LEFT JOIN
+          external_tags et
+          ON
+            et.id = ntv.tag_id
+      WHERE
+        ntv.version_no = 1
+        AND et.id IS NULL
+    SQL
+  end
+
+  def down
+    drop_table :external_tags
+  end
+end
diff --git a/backend/db/migrate/20260921050000_create_post_external_tags.rb b/backend/db/migrate/20260921050000_create_post_external_tags.rb
new file mode 100644
index 0000000..3b2efc5
--- /dev/null
+++ b/backend/db/migrate/20260921050000_create_post_external_tags.rb
@@ -0,0 +1,29 @@
+class CreatePostExternalTags < ActiveRecord::Migration[8.0]
+  def up
+    create_table :post_external_tags, primary_key: [:post_id, :external_tag_id] do |t|
+      t.references :post, null: false, index: false, foreign_key: true
+      t.references :external_tag, null: false, foreign_key: true
+      t.datetime :created_at, null: false
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        post_external_tags(post_id, external_tag_id, created_at)
+      SELECT
+        pt.post_id
+      , pt.tag_id AS external_tag_id
+      , pt.created_at
+      FROM
+        post_tags pt
+        INNER JOIN
+          tags t
+          ON
+            pt.tag_id = t.id
+            AND t.category = 'nico'
+    SQL
+  end
+
+  def down
+    drop_table :post_external_tags
+  end
+end
diff --git a/backend/db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations.rb b/backend/db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations.rb
new file mode 100644
index 0000000..ac24e0b
--- /dev/null
+++ b/backend/db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations.rb
@@ -0,0 +1,6 @@
+class ChangeForeignKeyOnNicoTagRelations < ActiveRecord::Migration[8.0]
+  def change
+    remove_foreign_key :nico_tag_relations, :tags, column: :nico_tag_id
+    add_foreign_key :nico_tag_relations, :external_tags, column: :nico_tag_id
+  end
+end
diff --git a/backend/db/migrate/20260921230000_migrate_external_tags.rb b/backend/db/migrate/20260921230000_migrate_external_tags.rb
new file mode 100644
index 0000000..f410d67
--- /dev/null
+++ b/backend/db/migrate/20260921230000_migrate_external_tags.rb
@@ -0,0 +1,199 @@
+class MigrateExternalTags < ActiveRecord::Migration[8.0]
+  class MigrationPostVersion < ActiveRecord::Base
+    self.table_name = 'post_versions'
+  end
+
+  def up
+    x = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        post_tag_sections pts
+      INNER JOIN
+        tags t
+        ON
+          t.id = pts.tag_id
+      WHERE
+        t.category = 'nico'
+    SQL
+    if x > 0
+      raise "post_tag_sections に #{ x } 件のチンカスがあります!"
+    end
+
+    x = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        materials m
+        INNER JOIN
+          tags t
+          ON
+            t.id = m.tag_id
+      WHERE
+        t.category = 'nico'
+    SQL
+    if x > 0
+      raise "materials に #{ x } 件のチンカスがあります!"
+    end
+
+    x = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        tag_implications ti
+        INNER JOIN
+          tags t
+          ON
+            t.id = ti.tag_id
+            OR t.id = ti.parent_tag_id
+      WHERE
+        t.category = 'nico'
+    SQL
+    if x > 0
+      raise "tag_implications に #{ x } 件のチンカスがあります!"
+    end
+
+    execute <<~SQL
+      DELETE
+        ts
+      FROM
+        tag_similarities ts
+        INNER JOIN
+          tags t
+          ON
+            t.category = 'nico'
+            AND (t.id = ts.tag_id
+                 OR t.id = ts.target_tag_id)
+    SQL
+
+    execute <<~SQL
+      DELETE
+        tset
+      FROM
+        theatre_skip_event_tags tset
+        INNER JOIN
+          tags t
+          ON
+            t.category = 'nico'
+            AND t.id = tset.tag_id
+    SQL
+
+    remove_check_constraint :post_versions, name: 'chk_post_versions_tags_json_schema'
+
+    say_with_time 'Migrate post_versions.tags_json' do
+      count = 0
+
+      MigrationPostVersion.find_each(batch_size: 500) do |version|
+        tags = version.tags_json.map do |tag|
+          if tag.fetch('category') == 'nico'
+            { 'external_tag_id' => tag.fetch('id') }
+          else
+            { 'tag_id' => tag.fetch('id'),
+              'version_no' => tag.fetch('version_no'),
+              'name' => tag.fetch('name'),
+              'category' => tag.fetch('category'),
+              'sections' => tag.fetch('sections') }
+          end
+        end
+
+        version.update_columns(tags_json: tags)
+        count += 1
+      end
+
+      count
+    end
+
+    add_tags_json_constraint!
+
+    tag_name_ids = connection.select_values(<<~SQL)
+      SELECT
+        tag_name_id
+      FROM
+        tags
+      WHERE
+        category = 'nico'
+    SQL
+
+    connection.transaction do
+      execute <<~SQL
+        DELETE
+          pt
+        FROM
+          post_tags pt
+          INNER JOIN
+            tags t
+            ON
+              t.category = 'nico'
+              AND t.id = pt.tag_id
+      SQL
+
+      execute <<~SQL
+        DELETE
+        FROM
+          tags
+        WHERE
+          category = 'nico'
+      SQL
+
+      unless tag_name_ids.empty?
+        execute <<~SQL
+          DELETE
+          FROM
+            tag_names
+          WHERE
+            id IN (#{ tag_name_ids.join(', ') })
+        SQL
+      end
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+
+  private
+
+  def add_tags_json_constraint!
+    schema = { type: 'array',
+               items: { oneOf: [internal_tag_schema, external_tag_schema] } }
+
+    quoted_schema = connection.quote(JSON.generate(schema))
+
+    add_check_constraint :post_versions,
+                         "JSON_SCHEMA_VALID(#{ quoted_schema }, tags_json)",
+                         name: 'chk_post_versions_tags_json_schema'
+  end
+
+  def internal_tag_schema
+    { type: 'object',
+      properties: { tag_id: { type: 'integer', minimum: 1 },
+                    version_no: { type: 'integer', minimum: 1 },
+                    name: { type: 'string', minLength: 1 },
+                    category: { type: 'string',
+                                enum: ['deerjikist',
+                                       'meme',
+                                       'character',
+                                       'general',
+                                       'material',
+                                       'meta'] },
+                    sections: sections_schema },
+      required: ['tag_id', 'version_no', 'sections'],
+      additionalProperties: false }
+  end
+
+  def external_tag_schema
+    { type: 'object',
+      properties: { external_tag_id: { type: 'integer', minimum: 1 } },
+      required: ['external_tag_id'],
+      additionalProperties: false }
+  end
+
+  def sections_schema
+    { type: 'array',
+      items: { type: 'object',
+               properties: { begin_ms: { type: 'integer', minimum: 0 },
+                             end_ms: { type: ['integer', 'null'], minimum: 0 } },
+               required: ['begin_ms', 'end_ms'],
+               additionalProperties: false } }
+  end
+end