diff --git a/include/openssl/base.h b/include/openssl/base.h
index 7fe232f..41e9305 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -291,6 +291,23 @@
 #endif
 #endif  // OPENSSL_ASM_INCOMPATIBLE
 
+#if defined(__cplusplus)
+// enums can be predeclared, but only in C++ and only if given an explicit type.
+// C doesn't support setting an explicit type for enums thus a #define is used
+// to do this only for C++. However, the ABI type between C and C++ need to have
+// equal sizes, which is confirmed in a unittest.
+#define BORINGSSL_ENUM_INT : int
+enum ssl_private_key_result_t BORINGSSL_ENUM_INT;
+enum ssl_ticket_aead_result_t BORINGSSL_ENUM_INT;
+enum ssl_verify_result_t BORINGSSL_ENUM_INT;
+enum ssl_encryption_level_t BORINGSSL_ENUM_INT;
+enum ssl_renegotiate_mode_t BORINGSSL_ENUM_INT;
+enum ssl_select_cert_result_t BORINGSSL_ENUM_INT;
+enum ssl_select_cert_result_t BORINGSSL_ENUM_INT;
+#else
+#define BORINGSSL_ENUM_INT
+#endif
+
 // CRYPTO_THREADID is a dummy value.
 typedef int CRYPTO_THREADID;
 
diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 4240c29..8e02084 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -1160,7 +1160,7 @@
 
 // Custom private keys.
 
-enum ssl_private_key_result_t {
+enum ssl_private_key_result_t BORINGSSL_ENUM_INT {
   ssl_private_key_success,
   ssl_private_key_retry,
   ssl_private_key_failure,
@@ -2111,7 +2111,7 @@
 
 // ssl_ticket_aead_result_t enumerates the possible results from decrypting a
 // ticket with an |SSL_TICKET_AEAD_METHOD|.
-enum ssl_ticket_aead_result_t {
+enum ssl_ticket_aead_result_t BORINGSSL_ENUM_INT {
   // ssl_ticket_aead_success indicates that the ticket was successfully
   // decrypted.
   ssl_ticket_aead_success,
@@ -2285,7 +2285,7 @@
                                    int (*callback)(int ok,
                                                    X509_STORE_CTX *store_ctx));
 
-enum ssl_verify_result_t {
+enum ssl_verify_result_t BORINGSSL_ENUM_INT {
   ssl_verify_ok,
   ssl_verify_invalid,
   ssl_verify_retry,
@@ -3135,7 +3135,7 @@
 
 // ssl_encryption_level_t represents a specific QUIC encryption level used to
 // transmit handshake messages.
-enum ssl_encryption_level_t {
+enum ssl_encryption_level_t BORINGSSL_ENUM_INT {
   ssl_encryption_initial = 0,
   ssl_encryption_early_data,
   ssl_encryption_handshake,
@@ -3522,7 +3522,7 @@
 // such as HTTP/1.1, and not others, such as HTTP/2.
 OPENSSL_EXPORT void SSL_set_shed_handshake_config(SSL *ssl, int enable);
 
-enum ssl_renegotiate_mode_t {
+enum ssl_renegotiate_mode_t BORINGSSL_ENUM_INT {
   ssl_renegotiate_never = 0,
   ssl_renegotiate_once,
   ssl_renegotiate_freely,
@@ -3620,7 +3620,7 @@
 
 // ssl_select_cert_result_t enumerates the possible results from selecting a
 // certificate with |select_certificate_cb|.
-enum ssl_select_cert_result_t {
+enum ssl_select_cert_result_t BORINGSSL_ENUM_INT {
   // ssl_select_cert_success indicates that the certificate selection was
   // successful.
   ssl_select_cert_success = 1,
diff --git a/ssl/CMakeLists.txt b/ssl/CMakeLists.txt
index dc89dca..0fb532e 100644
--- a/ssl/CMakeLists.txt
+++ b/ssl/CMakeLists.txt
@@ -50,6 +50,7 @@
 
   span_test.cc
   ssl_test.cc
+  ssl_c_test.c
 
   $<TARGET_OBJECTS:boringssl_gtest_main>
 )
diff --git a/ssl/ssl_c_test.c b/ssl/ssl_c_test.c
new file mode 100644
index 0000000..02f8655
--- /dev/null
+++ b/ssl/ssl_c_test.c
@@ -0,0 +1,15 @@
+#include <openssl/ssl.h>
+
+int BORINGSSL_enum_c_type_test(void);
+
+int BORINGSSL_enum_c_type_test(void) {
+#if defined(__cplusplus)
+#error "This is testing the behaviour of the C compiler."
+#error "It's pointless to build it in C++ mode."
+#endif
+
+  // In C++, the enums in ssl.h are explicitly typed as ints to allow them to
+  // be predeclared. This function confirms that the C compiler believes them
+  // to be the same size as ints. They may differ in signedness, however.
+  return sizeof(enum ssl_private_key_result_t) == sizeof(int);
+}
diff --git a/ssl/ssl_test.cc b/ssl/ssl_test.cc
index d01b649..0ff1867 100644
--- a/ssl/ssl_test.cc
+++ b/ssl/ssl_test.cc
@@ -5263,6 +5263,15 @@
   EXPECT_EQ(SSL_process_quic_post_handshake(client_.get()), 0);
 }
 
+extern "C" {
+int BORINGSSL_enum_c_type_test(void);
+}
+
+TEST(SSLTest, EnumTypes) {
+  EXPECT_EQ(sizeof(int), sizeof(ssl_private_key_result_t));
+  EXPECT_EQ(1, BORINGSSL_enum_c_type_test());
+}
+
 // TODO(davidben): Convert this file to GTest properly.
 TEST(SSLTest, AllTests) {
   if (!TestSSL_SESSIONEncoding(kOpenSSLSession) ||