diff --git a/esphome/core/config.py b/esphome/core/config.py
index 923063c694..368c1b73ca 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -273,7 +273,7 @@ def preload_core_config(config, result) -> str:
                 )
 
     if has_oldstyle:
-        plat = conf.pop(CONF_PLATFORM)
+        plat = conf.pop(CONF_PLATFORM).lower()
         if not _is_target_platform(plat):
             raise cv.Invalid(
                 "Platform missing. You must include one of the available platform keys: "
diff --git a/tests/test_build_components/build_components_base.esp32-oldstyle.yaml b/tests/test_build_components/build_components_base.esp32-oldstyle.yaml
index 3f64125f82..a1adf1b934 100644
--- a/tests/test_build_components/build_components_base.esp32-oldstyle.yaml
+++ b/tests/test_build_components/build_components_base.esp32-oldstyle.yaml
@@ -1,7 +1,7 @@
 esphome:
   name: componenttestesp32oldstyle
   friendly_name: $component_name
-  platform: esp32
+  platform: ESP32
   board: nodemcu-32s
 
 logger: