diff --git a/include/openssl/base.h b/include/openssl/base.h
index 7afcc4f..1e61e98 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -326,6 +326,15 @@
 #define BORINGSSL_ENUM_INT
 #endif
 
+// ossl_ssize_t is a signed type which is large enough to fit the size of any
+// valid memory allocation. We prefer using |size_t|, but sometimes we need a
+// signed type for OpenSSL API compatibility. This type can be used in such
+// cases to avoid overflow.
+//
+// Not all |size_t| values fit in |ossl_ssize_t|, but all |size_t| values that
+// are sizes of or indices into C objects, can be converted without overflow.
+typedef ptrdiff_t ossl_ssize_t;
+
 // CRYPTO_THREADID is a dummy value.
 typedef int CRYPTO_THREADID;