diff --git a/esphome/components/modem/__init__.py b/esphome/components/modem/__init__.py
index 9d59e43fca..7d87c213e7 100644
--- a/esphome/components/modem/__init__.py
+++ b/esphome/components/modem/__init__.py
@@ -39,7 +39,7 @@ CONFIG_SCHEMA = cv.All(
             cv.Required(CONF_RX_PIN): cv.positive_int,
             cv.Required(CONF_MODEL): cv.string,
             cv.Required(CONF_APN): cv.string,
-            cv.Optional(CONF_READY): cv.use_id(BinarySensor),
+            cv.Required(CONF_READY): cv.use_id(BinarySensor),
             cv.Optional(CONF_FLIGHT_PIN): cv.positive_int,
             cv.Optional(CONF_POWER_PIN): cv.positive_int,
             cv.Optional(CONF_STATUS_PIN): cv.positive_int,