diff --git a/esphome/components/esphome/ota/ota_esphome.cpp b/esphome/components/esphome/ota/ota_esphome.cpp
index aeaa3f9026..955b4dc96f 100644
--- a/esphome/components/esphome/ota/ota_esphome.cpp
+++ b/esphome/components/esphome/ota/ota_esphome.cpp
@@ -212,6 +212,7 @@ void ESPHomeOTAComponent::handle_handshake_() {
       // legacy response.
       this->extended_proto_ = (this->ota_features_ & CLIENT_FEATURE_SUPPORTS_EXTENDED_PROTOCOL) != 0;
       if (this->extended_proto_) {
+        static_assert(HANDSHAKE_BUF_SIZE >= 2, "handshake_buf_ must hold the 2-byte extended-protocol feature ack");
         this->handshake_buf_[0] = ota::OTA_RESPONSE_FEATURE_FLAGS;
         this->handshake_buf_[1] = (supports_compression ? SERVER_FEATURE_SUPPORTS_COMPRESSION : 0);
       } else {
diff --git a/esphome/components/esphome/ota/ota_esphome.h b/esphome/components/esphome/ota/ota_esphome.h
index 0637374ee4..5043bc33ef 100644
--- a/esphome/components/esphome/ota/ota_esphome.h
+++ b/esphome/components/esphome/ota/ota_esphome.h
@@ -97,8 +97,9 @@ class ESPHomeOTAComponent final : public ota::OTAComponent {
   ota::OTABackendPtr backend_;
 
   uint32_t client_connect_time_{0};
+  static constexpr size_t HANDSHAKE_BUF_SIZE = 5;
   uint16_t port_;
-  uint8_t handshake_buf_[5];
+  uint8_t handshake_buf_[HANDSHAKE_BUF_SIZE];
   OTAState ota_state_{OTAState::IDLE};
   uint8_t handshake_buf_pos_{0};
   uint8_t ota_features_{0};