diff --git a/CHANGELOG.md b/CHANGELOG.md
index fdc7d3a3e..9dda411fc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@
 * [#2918](https://github.com/ruby-grape/grape/pull/2918): Skip the dry-types round trip when a value already is the declared type - [@ericproulx](https://github.com/ericproulx).
 * [#2917](https://github.com/ruby-grape/grape/pull/2917): Read path captures out of the router's union match instead of re-running the route's pattern - [@ericproulx](https://github.com/ericproulx).
 * [#2921](https://github.com/ruby-grape/grape/pull/2921): Pin the router's request-time isolation regressions through requests instead of its instance variables - [@ericproulx](https://github.com/ericproulx).
+* [#2926](https://github.com/ruby-grape/grape/pull/2926): Skip the renamed-params lookup in `declared` when nothing is renamed - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 4.0.0 (2026-09-07)
diff --git a/lib/grape/declared_params_handler.rb b/lib/grape/declared_params_handler.rb
index 5191a49d1..b8d57b4d3 100644
--- a/lib/grape/declared_params_handler.rb
+++ b/lib/grape/declared_params_handler.rb
@@ -87,8 +87,11 @@ def declare_leaf(passed_params, declared_param:, params_nested_path:, memo:, ren
       end
     end
 
+    # The lookup key is the param's whole path, built fresh -- two Arrays and a
+    # String per declared param -- and only an API using +as:+ has anything to
+    # find with it, so the common empty table is not asked.
     def build_memo_key(params_nested_path, declared_param, renamed_params)
-      renamed_param_name = renamed_params[nested_path_for(params_nested_path, declared_param)]
+      renamed_param_name = renamed_params[nested_path_for(params_nested_path, declared_param)] unless renamed_params.empty?
       param = renamed_param_name || declared_param
       @stringify ? param.to_s : param.to_sym
     end