diff --git a/lib/Driver/ToolChains.cpp b/lib/Driver/ToolChains.cpp
index d01e31f..c42b08c 100644
--- a/lib/Driver/ToolChains.cpp
+++ b/lib/Driver/ToolChains.cpp
@@ -510,6 +510,27 @@
   }
 }
 
+/// Returns the most appropriate macOS target version for the current process.
+///
+/// If the macOS SDK version is the same or earlier than the system version,
+/// then the SDK version is returned. Otherwise the system version is returned.
+static std::string getSystemOrSDKMacOSVersion(StringRef MacOSSDKVersion) {
+  unsigned Major, Minor, Micro;
+  llvm::Triple SystemTriple(llvm::sys::getProcessTriple());
+  if (!SystemTriple.isMacOSX())
+    return MacOSSDKVersion;
+  SystemTriple.getMacOSXVersion(Major, Minor, Micro);
+  VersionTuple SystemVersion(Major, Minor, Micro);
+  bool HadExtra;
+  if (!Driver::GetReleaseVersion(MacOSSDKVersion, Major, Minor, Micro,
+                                 HadExtra))
+    return MacOSSDKVersion;
+  VersionTuple SDKVersion(Major, Minor, Micro);
+  if (SDKVersion > SystemVersion)
+    return SystemVersion.getAsString();
+  return MacOSSDKVersion;
+}
+
 void Darwin::AddDeploymentTarget(DerivedArgList &Args) const {
   const OptTable &Opts = getDriver().getOpts();
 
@@ -602,7 +623,7 @@
                 SDK.startswith("iPhoneSimulator"))
               iOSTarget = Version;
             else if (SDK.startswith("MacOSX"))
-              OSXTarget = Version;
+              OSXTarget = getSystemOrSDKMacOSVersion(Version);
             else if (SDK.startswith("WatchOS") ||
                      SDK.startswith("WatchSimulator"))
               WatchOSTarget = Version;
diff --git a/test/Driver/darwin-sdk-vs-os-version.c b/test/Driver/darwin-sdk-vs-os-version.c
new file mode 100644
index 0000000..391f4d5
--- /dev/null
+++ b/test/Driver/darwin-sdk-vs-os-version.c
@@ -0,0 +1,10 @@
+// REQUIRES: system-darwin
+
+// Ensure that we never pick a version that's based on the SDK that's newer than
+// the system version:
+// RUN: rm -rf %t/SDKs/MacOSX10.99.99.sdk
+// RUN: mkdir -p %t/SDKs/MacOSX10.99.99.sdk
+// RUN: %clang -target x86_64-apple-darwin -isysroot %t/SDKs/MacOSX10.99.99.sdk %s -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-MACOSX-SYSTEM-VERSION %s
+
+// CHECK-MACOSX-SYSTEM-VERSION-NOT: 10.99.99"