diff --git a/e_os2.h b/e_os2.h
index c9f2543e84..9d691e2819 100644
--- a/e_os2.h
+++ b/e_os2.h
@@ -109,6 +109,12 @@ extern "C" {
 #    undef OPENSSL_SYS_UNIX
 #    define OPENSSL_SYS_WIN32
 #   endif
+#   if defined(_WIN64) || defined(OPENSSL_SYSNAME_WIN64)
+#    undef OPENSSL_SYS_UNIX
+#    if !defined(OPENSSL_SYS_WIN64)
+#     define OPENSSL_SYS_WIN64
+#    endif
+#   endif
 #   if defined(OPENSSL_SYSNAME_WINNT)
 #    undef OPENSSL_SYS_UNIX
 #    define OPENSSL_SYS_WINNT
@@ -121,7 +127,7 @@ extern "C" {
 # endif
 
 /* Anything that tries to look like Microsoft is "Windows" */
-# if defined(OPENSSL_SYS_WIN32) || defined(OPENSSL_SYS_WINNT) || defined(OPENSSL_SYS_WINCE)
+# if defined(OPENSSL_SYS_WIN32) || defined(OPENSSL_SYS_WIN64) || defined(OPENSSL_SYS_WINNT) || defined(OPENSSL_SYS_WINCE)
 #  undef OPENSSL_SYS_UNIX
 #  define OPENSSL_SYS_WINDOWS
 #  ifndef OPENSSL_SYS_MSDOS