diff --git a/esphome/core/config.py b/esphome/core/config.py
index f4253bee87..8c130eb6db 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -318,6 +318,8 @@ async def add_includes(includes):
 async def _add_platformio_options(pio_options):
     # Add includes at the very end, so that they override everything
     for key, val in pio_options.items():
+        if key == "build_flags" and not isinstance(val, list):
+            val = [val]
         cg.add_platformio_option(key, val)