diff --git a/db/docs/spam_logs.yml b/db/docs/spam_logs.yml
index 6738fe6f613bd6dabec41ec828f81d46555ad35b..a09f678dc6eb9f69198da2598043de589a9b3b03 100644
--- a/db/docs/spam_logs.yml
+++ b/db/docs/spam_logs.yml
@@ -8,14 +8,6 @@ description: Logs users flagged by the Akismet anti-spam integration.
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/2266
 milestone: '8.5'
 gitlab_schema: gitlab_main_org
-desired_sharding_key:
-  organization_id:
-    references: organizations
-    backfill_via:
-      parent:
-        foreign_key: user_id
-        table: users
-        table_primary_key: id
-        sharding_key: organization_id
-        belongs_to: user
+sharding_key:
+  organization_id: organizations
 table_size: small
diff --git a/db/migrate/20251112163007_add_sharding_key_trigger_to_spam_logs.rb b/db/migrate/20251112163007_add_sharding_key_trigger_to_spam_logs.rb
new file mode 100644
index 0000000000000000000000000000000000000000..cc39bfc484cbdf590cd0f54913a2cb6a2c45dd26
--- /dev/null
+++ b/db/migrate/20251112163007_add_sharding_key_trigger_to_spam_logs.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class AddShardingKeyTriggerToSpamLogs < Gitlab::Database::Migration[2.3]
+  milestone '18.6'
+
+  TABLE_NAME = :spam_logs
+  SHARDING_KEY = :organization_id
+  PARENT_TABLE = :users
+  PARENT_SHARDING_KEY = :organization_id
+  FOREIGN_KEY = :user_id
+
+  def up
+    install_sharding_key_assignment_trigger(
+      table: TABLE_NAME,
+      sharding_key: SHARDING_KEY,
+      parent_table: PARENT_TABLE,
+      parent_sharding_key: PARENT_SHARDING_KEY,
+      foreign_key: FOREIGN_KEY
+    )
+  end
+
+  def down
+    remove_sharding_key_assignment_trigger(
+      table: TABLE_NAME,
+      sharding_key: SHARDING_KEY,
+      parent_table: PARENT_TABLE,
+      parent_sharding_key: PARENT_SHARDING_KEY,
+      foreign_key: FOREIGN_KEY
+    )
+  end
+end
diff --git a/db/migrate/20251112163329_add_not_null_to_spam_logs_on_organization_id.rb b/db/migrate/20251112163329_add_not_null_to_spam_logs_on_organization_id.rb
new file mode 100644
index 0000000000000000000000000000000000000000..9854ad4d49f02bcde7c721b51faac50bcc84a66a
--- /dev/null
+++ b/db/migrate/20251112163329_add_not_null_to_spam_logs_on_organization_id.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class AddNotNullToSpamLogsOnOrganizationId < Gitlab::Database::Migration[2.3]
+  disable_ddl_transaction!
+  milestone '18.6'
+
+  def up
+    add_not_null_constraint :spam_logs, :organization_id, validate: false
+  end
+
+  def down
+    remove_not_null_constraint :spam_logs, :organization_id
+  end
+end
diff --git a/db/post_migrate/20251112163549_backfill_spam_logs_organization_id.rb b/db/post_migrate/20251112163549_backfill_spam_logs_organization_id.rb
new file mode 100644
index 0000000000000000000000000000000000000000..87539adcd6d56da8ce0f8859b0d255797e1036a9
--- /dev/null
+++ b/db/post_migrate/20251112163549_backfill_spam_logs_organization_id.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class BackfillSpamLogsOrganizationId < Gitlab::Database::Migration[2.3]
+  restrict_gitlab_migration gitlab_schema: :gitlab_main_org
+  disable_ddl_transaction!
+  milestone '18.6'
+
+  DEFAULT_ORGANIZATION_ID = 1
+
+  def up
+    define_batchable_model(:spam_logs).each_batch(of: 500) do |batch|
+      # Use default organization for rows that have a NULL user_id
+      batch
+        .where(organization_id: nil)
+        .where(user_id: nil)
+        .update_all(organization_id: DEFAULT_ORGANIZATION_ID)
+
+      # Trigger trigger_0e7036da74c7() to backfill organization_id
+      batch
+        .where.not(user_id: nil)
+        .where(organization_id: nil)
+        .update_all('updated_at = updated_at')
+    end
+  end
+
+  def down; end
+end
diff --git a/db/post_migrate/20251112165028_validate_organization_id_on_spam_logs.rb b/db/post_migrate/20251112165028_validate_organization_id_on_spam_logs.rb
new file mode 100644
index 0000000000000000000000000000000000000000..c392aeaa31368b232130ee38dba23d45cdef3c80
--- /dev/null
+++ b/db/post_migrate/20251112165028_validate_organization_id_on_spam_logs.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ValidateOrganizationIdOnSpamLogs < Gitlab::Database::Migration[2.3]
+  milestone '18.6'
+
+  def up
+    validate_not_null_constraint :spam_logs, :organization_id
+  end
+
+  def down; end
+end
diff --git a/db/schema_migrations/20251112163007 b/db/schema_migrations/20251112163007
new file mode 100644
index 0000000000000000000000000000000000000000..d1845a8e72b0de6c832267d7a8c8d82d608d8a51
--- /dev/null
+++ b/db/schema_migrations/20251112163007
@@ -0,0 +1 @@
+45a5009e100358df8bc505591389b8953005b5381122b740ae845276c11c5581
\ No newline at end of file
diff --git a/db/schema_migrations/20251112163329 b/db/schema_migrations/20251112163329
new file mode 100644
index 0000000000000000000000000000000000000000..4d4cf26b0b1f8aea9ff8814320d47d1342746fc6
--- /dev/null
+++ b/db/schema_migrations/20251112163329
@@ -0,0 +1 @@
+4256db6d66962af085f88429ada25183e9a985c2293c5358eb8ea87d2a355930
\ No newline at end of file
diff --git a/db/schema_migrations/20251112163549 b/db/schema_migrations/20251112163549
new file mode 100644
index 0000000000000000000000000000000000000000..d77a5a66cdc8b8593f20f4e08c70642fbf7e356b
--- /dev/null
+++ b/db/schema_migrations/20251112163549
@@ -0,0 +1 @@
+c55f110ca4e415443725be4913265571f1c8328ecfcc56536bceb7cd11d35377
\ No newline at end of file
diff --git a/db/schema_migrations/20251112165028 b/db/schema_migrations/20251112165028
new file mode 100644
index 0000000000000000000000000000000000000000..59de3efa6f0c1cfdd1fb713235b31a62301c9e65
--- /dev/null
+++ b/db/schema_migrations/20251112165028
@@ -0,0 +1 @@
+ce534f52d3a7682295c3aaec93a7256b959eb73d58eb2f60d0561586c190466a
\ No newline at end of file
diff --git a/db/structure.sql b/db/structure.sql
index e95a3aaa2961d9a07fce585708c20b17d4df787f..49e10c44fa568c41bd0ed231749c0ce83fad24e6 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -1696,6 +1696,22 @@ RETURN NEW;
 END
 $$;
 
+CREATE FUNCTION trigger_0e7036da74c7() RETURNS trigger
+    LANGUAGE plpgsql
+    AS $$
+BEGIN
+IF NEW."organization_id" IS NULL THEN
+  SELECT "organization_id"
+  INTO NEW."organization_id"
+  FROM "users"
+  WHERE "users"."id" = NEW."user_id";
+END IF;
+
+RETURN NEW;
+
+END
+$$;
+
 CREATE FUNCTION trigger_0f38e5af9adf() RETURNS trigger
     LANGUAGE plpgsql
     AS $$
@@ -26693,7 +26709,8 @@ CREATE TABLE spam_logs (
     submitted_as_ham boolean DEFAULT false NOT NULL,
     recaptcha_verified boolean DEFAULT false NOT NULL,
     target_id bigint,
-    organization_id bigint
+    organization_id bigint,
+    CONSTRAINT check_0c0873a24a CHECK ((organization_id IS NOT NULL))
 );
 
 CREATE SEQUENCE spam_logs_id_seq
@@ -47605,6 +47622,8 @@ CREATE TRIGGER trigger_0ddb594934c9 BEFORE INSERT OR UPDATE ON incident_manageme
 
 CREATE TRIGGER trigger_0e13f214e504 BEFORE INSERT OR UPDATE ON merge_request_assignment_events FOR EACH ROW EXECUTE FUNCTION trigger_0e13f214e504();
 
+CREATE TRIGGER trigger_0e7036da74c7 BEFORE INSERT OR UPDATE ON spam_logs FOR EACH ROW EXECUTE FUNCTION trigger_0e7036da74c7();
+
 CREATE TRIGGER trigger_0f38e5af9adf BEFORE INSERT OR UPDATE ON ml_candidate_params FOR EACH ROW EXECUTE FUNCTION trigger_0f38e5af9adf();
 
 CREATE TRIGGER trigger_13d4aa8fe3dd BEFORE INSERT OR UPDATE ON draft_notes FOR EACH ROW EXECUTE FUNCTION trigger_13d4aa8fe3dd();
diff --git a/spec/migrations/backfill_spam_logs_organization_id_spec.rb b/spec/migrations/backfill_spam_logs_organization_id_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..b8a02bec4f426890f3bb544726f59cb68fb96553
--- /dev/null
+++ b/spec/migrations/backfill_spam_logs_organization_id_spec.rb
@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require_migration!
+
+RSpec.describe BackfillSpamLogsOrganizationId, feature_category: :instance_resiliency do
+  let(:connection) { ApplicationRecord.connection }
+  let(:spam_logs) { table(:spam_logs) }
+  let(:organizations) { table(:organizations) }
+  let(:users) { table(:users) }
+  let(:trigger_name) { 'trigger_0e7036da74c7' }
+  let(:constraint_name) { 'check_0c0873a24a' }
+
+  let!(:default_organization) { organizations.create!(id: 1, name: 'default', path: 'default') }
+  let!(:organization) { organizations.create!(name: 'org', path: 'org') }
+  let!(:user1) { create_user('user1') }
+  let!(:user2) { create_user('user2') }
+
+  let!(:spam_log_with_org) { create_spam_log(user1, organization) }
+
+  let!(:spam_log_without_org) do
+    drop_constraint_and_trigger
+    create_spam_log(user2, nil)
+  end
+
+  let!(:spam_log_without_org_or_user) do
+    drop_constraint_and_trigger
+    create_spam_log(nil, nil)
+  end
+
+  before do
+    recreate_trigger
+  end
+
+  after do
+    recreate_constraint
+  end
+
+  it 'backfills with nil organization_id' do
+    expect(spam_logs.where(organization_id: nil).count).to eq(2)
+    expect(spam_logs.where.not(organization_id: nil).count).to eq(1)
+
+    expect { migrate! }.to change { spam_logs.where(organization_id: nil).count }.by(-2)
+
+    expect(spam_logs.find_by(id: spam_log_with_org.id).organization_id).to eq(organization.id)
+    expect(spam_logs.find_by(id: spam_log_without_org.id).organization_id).to eq(organization.id)
+    expect(spam_logs.find_by(id: spam_log_without_org_or_user.id).organization_id).to eq(
+      Organizations::Organization::DEFAULT_ORGANIZATION_ID
+    )
+  end
+
+  private
+
+  def create_user(username)
+    users.create!(
+      email: "#{username}@example.com",
+      username: username,
+      organization_id: organization.id,
+      projects_limit: 10
+    )
+  end
+
+  def create_spam_log(user, organization)
+    spam_logs.create!(
+      user_id: user&.id,
+      organization_id: organization&.id
+    )
+  end
+
+  def drop_constraint_and_trigger
+    connection.execute(
+      <<~SQL
+        DROP TRIGGER IF EXISTS #{trigger_name} ON spam_logs;
+
+        ALTER TABLE spam_logs DROP CONSTRAINT IF EXISTS #{constraint_name};
+      SQL
+    )
+  end
+
+  def recreate_constraint
+    connection.execute(
+      <<~SQL
+        ALTER TABLE spam_logs ADD CONSTRAINT #{constraint_name} CHECK ((organization_id IS NOT NULL));
+      SQL
+    )
+  end
+
+  def recreate_trigger
+    connection.execute(
+      <<~SQL
+        CREATE TRIGGER #{trigger_name} BEFORE INSERT OR UPDATE ON spam_logs FOR EACH ROW EXECUTE FUNCTION #{trigger_name}();
+      SQL
+    )
+  end
+end