diff --git a/mkspecs/win32-msvc2005/qmake.conf b/mkspecs/win32-msvc2005/qmake.conf
index 3b01b77d38d..508db2a11ce 100644
--- a/mkspecs/win32-msvc2005/qmake.conf
+++ b/mkspecs/win32-msvc2005/qmake.conf
@@ -9,6 +9,10 @@ QMAKE_PLATFORM          = win32
 CONFIG                 += incremental flat precompile_header autogen_precompile_source debug_and_release debug_and_release_target embed_manifest_dll embed_manifest_exe
 DEFINES                += UNICODE WIN32
 QMAKE_COMPILER_DEFINES += _MSC_VER=1400 _WIN32
+contains(QMAKE_TARGET.arch, x86_64) {
+    DEFINES += WIN64
+    QMAKE_COMPILER_DEFINES += _WIN64
+}
 
 QMAKE_COMPILER          = msvc
 
diff --git a/mkspecs/win32-msvc2008/qmake.conf b/mkspecs/win32-msvc2008/qmake.conf
index eee56b6c601..7d1304aaed6 100644
--- a/mkspecs/win32-msvc2008/qmake.conf
+++ b/mkspecs/win32-msvc2008/qmake.conf
@@ -9,6 +9,10 @@ QMAKE_PLATFORM          = win32
 CONFIG                 += incremental flat precompile_header autogen_precompile_source debug_and_release debug_and_release_target embed_manifest_dll embed_manifest_exe
 DEFINES                += UNICODE WIN32
 QMAKE_COMPILER_DEFINES += _MSC_VER=1500 _WIN32
+contains(QMAKE_TARGET.arch, x86_64) {
+    DEFINES += WIN64
+    QMAKE_COMPILER_DEFINES += _WIN64
+}
 
 QMAKE_COMPILER          = msvc
 
diff --git a/mkspecs/win32-msvc2010/qmake.conf b/mkspecs/win32-msvc2010/qmake.conf
index a55bdb28af8..ebe043ceba3 100644
--- a/mkspecs/win32-msvc2010/qmake.conf
+++ b/mkspecs/win32-msvc2010/qmake.conf
@@ -9,6 +9,10 @@ QMAKE_PLATFORM          = win32
 CONFIG                 += incremental flat precompile_header autogen_precompile_source debug_and_release debug_and_release_target embed_manifest_dll embed_manifest_exe
 DEFINES                += UNICODE WIN32
 QMAKE_COMPILER_DEFINES += _MSC_VER=1600 _WIN32
+contains(QMAKE_TARGET.arch, x86_64) {
+    DEFINES += WIN64
+    QMAKE_COMPILER_DEFINES += _WIN64
+}
 
 QMAKE_COMPILER          = msvc
 
diff --git a/mkspecs/win32-msvc2012/qmake.conf b/mkspecs/win32-msvc2012/qmake.conf
index 56b8f04e24e..19bd10484b7 100644
--- a/mkspecs/win32-msvc2012/qmake.conf
+++ b/mkspecs/win32-msvc2012/qmake.conf
@@ -8,6 +8,10 @@ MAKEFILE_GENERATOR      = MSBUILD
 CONFIG                 += incremental flat precompile_header autogen_precompile_source debug_and_release debug_and_release_target embed_manifest_dll embed_manifest_exe
 DEFINES                += UNICODE WIN32
 QMAKE_COMPILER_DEFINES += _MSC_VER=1700 _WIN32
+contains(QMAKE_TARGET.arch, x86_64) {
+    DEFINES += WIN64
+    QMAKE_COMPILER_DEFINES += _WIN64
+}
 
 QMAKE_COMPILER          = msvc