diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index ca60a9d..3550ca0 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -1020,6 +1020,10 @@
 #error "MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT) && ( !defined(MBEDTLS_SSL_PROTO_TLS1_3) )
+#error "MBEDTLS_SSL_RECORD_SIZE_LIMIT defined, but not all prerequisites"
+#endif
+
 #if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION) && !( defined(MBEDTLS_GCM_C) || defined(MBEDTLS_CCM_C) || defined(MBEDTLS_CHACHAPOLY_C) )
 #error "MBEDTLS_SSL_CONTEXT_SERIALIZATION defined, but not all prerequisites"
 #endif
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 7daba37..509eeab 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -1548,6 +1548,20 @@
 #define MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 
 /**
+ * \def MBEDTLS_SSL_RECORD_SIZE_LIMIT
+ *
+ * Enable support for RFC 8449 record_size_limit extension in SSL.
+ *
+ * \warning This extension is currently in development and must NOT be used except
+ *          for testing purposes.
+ *
+ * Requires: MBEDTLS_SSL_PROTO_TLS1_3
+ *
+ * Uncomment this macro to enable support for the record_size_limit extension
+ */
+//#define MBEDTLS_SSL_RECORD_SIZE_LIMIT
+
+/**
  * \def MBEDTLS_SSL_PROTO_TLS1_2
  *
  * Enable support for TLS 1.2 (and DTLS 1.2 if DTLS is enabled).
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 29ba85a..efe0830 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -571,6 +571,8 @@
 #define MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC            22 /* 0x16 */
 #define MBEDTLS_TLS_EXT_EXTENDED_MASTER_SECRET  0x0017 /* 23 */
 
+#define MBEDTLS_TLS_EXT_RECORD_SIZE_LIMIT           28 /* RFC 8449 (implemented for TLS 1.3 only) */
+
 #define MBEDTLS_TLS_EXT_SESSION_TICKET              35
 
 #define MBEDTLS_TLS_EXT_PRE_SHARED_KEY              41 /* RFC 8446 TLS 1.3 */