diff --git a/lib/Basic/LangOptions.cpp b/lib/Basic/LangOptions.cpp
index 19efc6356c370..ae2bba6b0d8c6 100644
--- a/lib/Basic/LangOptions.cpp
+++ b/lib/Basic/LangOptions.cpp
@@ -35,6 +35,7 @@ static const StringRef SupportedConditionalCompilationOSs[] = {
   "Windows",
   "Android",
   "PS4",
+  "Cygwin"
 };
 
 static const StringRef SupportedConditionalCompilationArches[] = {
@@ -154,6 +155,8 @@ std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
     addPlatformConditionValue("os", "Linux");
   else if (triple.isOSFreeBSD())
     addPlatformConditionValue("os", "FreeBSD");
+  else if (triple.isWindowsCygwinEnvironment())
+    addPlatformConditionValue("os", "Cygwin");
   else if (triple.isOSWindows())
     addPlatformConditionValue("os", "Windows");
   else if (triple.isPS4())
diff --git a/test/Parse/ConditionalCompilation/x64CygwinTarget.swift b/test/Parse/ConditionalCompilation/x64CygwinTarget.swift
new file mode 100644
index 0000000000000..a5ac4ce055cb5
--- /dev/null
+++ b/test/Parse/ConditionalCompilation/x64CygwinTarget.swift
@@ -0,0 +1,8 @@
+// RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
+
+#if arch(x86_64) && os(Cygwin) && _runtime(_Native)
+class C {}
+var x = C()
+#endif
+var y = x
diff --git a/test/Parse/ConditionalCompilation/x64WindowsTarget.swift b/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
index 89f0f91b6833d..f89314c1ad6ef 100644
--- a/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
@@ -1,5 +1,5 @@
-// RUN: %swift -typecheck %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
-// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
+// RUN: %swift -typecheck %s -verify -D FOO -D BAR -target x86_64-unknown-windows-msvc -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-msvc
 
 #if arch(x86_64) && os(Windows) && _runtime(_Native) && _endian(little)
 class C {}