diff --git a/clang/lib/Driver/ToolChains/Darwin.h b/clang/lib/Driver/ToolChains/Darwin.h
--- a/clang/lib/Driver/ToolChains/Darwin.h
+++ b/clang/lib/Driver/ToolChains/Darwin.h
@@ -539,6 +539,8 @@
                                    llvm::StringRef Version,
                                    llvm::StringRef ArchDir,
                                    llvm::StringRef BitDir) const;
+
+  llvm::StringRef GetHeaderSysroot(const llvm::opt::ArgList &DriverArgs) const;
 };
 
 } // end namespace toolchains
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -1805,13 +1805,21 @@
   }
 }
 
+// Returns the effective header sysroot path to use. This comes either from
+// -isysroot or --sysroot.
+llvm::StringRef DarwinClang::GetHeaderSysroot(const llvm::opt::ArgList &DriverArgs) const {
+  if(DriverArgs.hasArg(options::OPT_isysroot))
+    return DriverArgs.getLastArgValue(options::OPT_isysroot);
+  if (!getDriver().SysRoot.empty())
+    return getDriver().SysRoot;
+  return "/";
+}
+
 void DarwinClang::AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                                             llvm::opt::ArgStringList &CC1Args) const {
   const Driver &D = getDriver();
 
-  llvm::StringRef Sysroot = "/";
-  if (const Arg *A = DriverArgs.getLastArg(options::OPT_isysroot))
-    Sysroot = A->getValue();
+  llvm::StringRef Sysroot = GetHeaderSysroot(DriverArgs);
 
   bool NoStdInc = DriverArgs.hasArg(options::OPT_nostdinc);
   bool NoStdlibInc = DriverArgs.hasArg(options::OPT_nostdlibinc);
@@ -1897,12 +1905,7 @@
       DriverArgs.hasArg(options::OPT_nostdincxx))
     return;
 
-  llvm::SmallString<128> Sysroot;
-  if (const Arg *A = DriverArgs.getLastArg(options::OPT_isysroot)) {
-    Sysroot = A->getValue();
-  } else {
-    Sysroot = "/";
-  }
+  llvm::StringRef Sysroot = GetHeaderSysroot(DriverArgs);
 
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
diff --git a/clang/test/Driver/darwin-header-search-libcxx.cpp b/clang/test/Driver/darwin-header-search-libcxx.cpp
--- a/clang/test/Driver/darwin-header-search-libcxx.cpp
+++ b/clang/test/Driver/darwin-header-search-libcxx.cpp
@@ -36,6 +36,7 @@
 
 // Check with both headers in the sysroot and headers alongside the installation
 // (the headers in <sysroot> should be added after the toolchain headers).
+// Ensure that both -isysroot and --sysroot work, and that isysroot has precedence.
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
@@ -46,6 +47,28 @@
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -stdlib=libc++ \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -stdlib=libc++ \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
+//
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
diff --git a/clang/test/Driver/darwin-header-search-system.cpp b/clang/test/Driver/darwin-header-search-system.cpp
--- a/clang/test/Driver/darwin-header-search-system.cpp
+++ b/clang/test/Driver/darwin-header-search-system.cpp
@@ -3,7 +3,8 @@
 // General tests that the system header search paths detected by the driver
 // and passed to CC1 are correct on Darwin platforms.
 
-// Check system headers (everything below <sysroot> and <resource-dir>)
+// Check system headers (everything below <sysroot> and <resource-dir>).  Ensure
+// that both sysroot and isysroot are checked, and that isysroot has precedence.
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
@@ -13,6 +14,26 @@
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_and_usr_local \
 // RUN:               -DRESOURCE=%S/Inputs/resource_dir \
 // RUN:               --check-prefix=CHECK-SYSTEM %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:               -DRESOURCE=%S/Inputs/resource_dir \
+// RUN:               --check-prefix=CHECK-SYSTEM %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:     --sysroot / \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:               -DRESOURCE=%S/Inputs/resource_dir \
+// RUN:               --check-prefix=CHECK-SYSTEM %s
+//
 // CHECK-SYSTEM: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-SYSTEM: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // CHECK-SYSTEM: "-internal-isystem" "[[RESOURCE]]/include"