diff --git a/examples/platforms/nrf52840/openthread-core-nrf52840-config.h b/examples/platforms/nrf52840/openthread-core-nrf52840-config.h
index 99980de38..ee3367f49 100644
--- a/examples/platforms/nrf52840/openthread-core-nrf52840-config.h
+++ b/examples/platforms/nrf52840/openthread-core-nrf52840-config.h
@@ -115,4 +115,12 @@
  */
 #define SETTINGS_CONFIG_PAGE_NUM                                4
 
+/**
+ * @def OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE
+ *
+ * The size of mbedTLS heap buffer when DTLS is enabled.
+ *
+ */
+#define OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE                     (4096 * sizeof(void *))
+
 #endif  // OPENTHREAD_CORE_NRF52840_CONFIG_H_
diff --git a/src/core/crypto/mbedtls.hpp b/src/core/crypto/mbedtls.hpp
index eb575d563..66bdf47ca 100644
--- a/src/core/crypto/mbedtls.hpp
+++ b/src/core/crypto/mbedtls.hpp
@@ -40,6 +40,8 @@
 #include <openthread-config.h>
 #endif
 
+#include <openthread-core-config.h>
+
 #include <mbedtls/memory_buffer_alloc.h>
 
 namespace Thread {
@@ -62,9 +64,9 @@ public:
     enum
     {
 #if OPENTHREAD_ENABLE_DTLS
-        kMemorySize = 2048 * sizeof(void *), ///< Size of memory buffer (bytes).
+        kMemorySize = OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE, ///< Size of memory buffer (bytes).
 #else
-        kMemorySize = 384,                   ///< Size of memory buffer (bytes).
+        kMemorySize = 384,                                 ///< Size of memory buffer (bytes).
 #endif
     };
 
diff --git a/src/core/openthread-core-default-config.h b/src/core/openthread-core-default-config.h
index c58ebf7a8..709eb5e5a 100644
--- a/src/core/openthread-core-default-config.h
+++ b/src/core/openthread-core-default-config.h
@@ -677,4 +677,14 @@
 #define OPENTHREAD_CONFIG_ENABLE_AUTO_START_SUPPORT             1
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE
+ *
+ * The size of mbedTLS heap buffer when DTLS is enabled.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE
+#define OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE                     (2048 * sizeof(void *))
+#endif
+
 #endif  // OPENTHREAD_CORE_DEFAULT_CONFIG_H_