diff --git a/changelogs/unreleased/index-namespaces-lower-name.yml b/changelogs/unreleased/index-namespaces-lower-name.yml
new file mode 100644
index 00000000000..ef08b6d6755
--- /dev/null
+++ b/changelogs/unreleased/index-namespaces-lower-name.yml
@@ -0,0 +1,5 @@
+---
+title: Add index on namespaces lower(name) for UsersController#exists
+merge_request:
+author:
+type: performance
diff --git a/db/migrate/20171220191323_add_index_on_namespaces_lower_name.rb b/db/migrate/20171220191323_add_index_on_namespaces_lower_name.rb
new file mode 100644
index 00000000000..7cf1d0cec68
--- /dev/null
+++ b/db/migrate/20171220191323_add_index_on_namespaces_lower_name.rb
@@ -0,0 +1,30 @@
+class AddIndexOnNamespacesLowerName < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+  INDEX_NAME = 'index_on_namespaces_lower_name'
+
+  disable_ddl_transaction!
+
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+    if Gitlab::Database.version.to_f >= 9.5
+      # Allow us to hot-patch the index manually ahead of the migration
+      execute "CREATE INDEX CONCURRENTLY IF NOT EXISTS #{INDEX_NAME} ON namespaces (lower(name));"
+    else
+      execute "CREATE INDEX CONCURRENTLY #{INDEX_NAME} ON namespaces (lower(name));"
+    end
+  end
+
+  def down
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+    if Gitlab::Database.version.to_f >= 9.2
+      execute "DROP INDEX CONCURRENTLY IF EXISTS #{INDEX_NAME};"
+    else
+      execute "DROP INDEX IF EXISTS #{INDEX_NAME};"
+    end
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 2b20628bd53..81b594cd0c1 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171219121201) do
+ActiveRecord::Schema.define(version: 20171220191323) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
diff --git a/lib/tasks/migrate/setup_postgresql.rake b/lib/tasks/migrate/setup_postgresql.rake
index 9cc986535e1..c9e3eed82f2 100644
--- a/lib/tasks/migrate/setup_postgresql.rake
+++ b/lib/tasks/migrate/setup_postgresql.rake
@@ -6,6 +6,7 @@ require Rails.root.join('db/migrate/20161212142807_add_lower_path_index_to_route
 require Rails.root.join('db/migrate/20170317203554_index_routes_path_for_like')
 require Rails.root.join('db/migrate/20170724214302_add_lower_path_index_to_redirect_routes')
 require Rails.root.join('db/migrate/20170503185032_index_redirect_routes_path_for_like')
+require Rails.root.join('db/migrate/20171220191323_add_index_on_namespaces_lower_name.rb')
 
 desc 'GitLab | Sets up PostgreSQL'
 task setup_postgresql: :environment do
@@ -15,4 +16,5 @@ task setup_postgresql: :environment do
   IndexRoutesPathForLike.new.up
   AddLowerPathIndexToRedirectRoutes.new.up
   IndexRedirectRoutesPathForLike.new.up
+  AddIndexOnNamespacesLowerName.new.up
 end