diff --git a/esphome/core/controller_registry.cpp b/esphome/core/controller_registry.cpp
index 92f23f5642..907e0f923d 100644
--- a/esphome/core/controller_registry.cpp
+++ b/esphome/core/controller_registry.cpp
@@ -6,8 +6,6 @@ namespace esphome {
 
 StaticVector<Controller *, CONTROLLER_REGISTRY_MAX> ControllerRegistry::controllers;
 
-void ControllerRegistry::register_controller(Controller *controller) { controllers.push_back(controller); }
-
 }  // namespace esphome
 
 #endif  // USE_CONTROLLER_REGISTRY
diff --git a/esphome/core/controller_registry.h b/esphome/core/controller_registry.h
index 3c7b2b83a8..b2122d3314 100644
--- a/esphome/core/controller_registry.h
+++ b/esphome/core/controller_registry.h
@@ -34,7 +34,7 @@ class ControllerRegistry {
    * Controllers should call this in their setup() method.
    * Typically only APIServer and WebServer register.
    */
-  static void register_controller(Controller *controller);
+  static void register_controller(Controller *controller) { controllers.push_back(controller); }
 
 // Notify method declarations (generated from entity_types.inc)
 // NOLINTBEGIN(bugprone-macro-parentheses)