diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index bbdd032e8a..e3eb34a846 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -85,14 +85,19 @@ module OS
     #      specifically been requested according to the rules above.
 
     def sdk(v = nil)
-      @locator ||= SDKLocator.new
+      @locator ||= if Xcode.without_clt?
+        XcodeSDKLocator.new
+      else
+        CLTSDKLocator.new
+      end
+
       begin
         sdk = if v.nil?
           (Xcode.version.to_i >= 7) ? @locator.latest_sdk : @locator.sdk_for(version)
         else
           @locator.sdk_for v
         end
-      rescue SDKLocator::NoSDKError
+      rescue BaseSDKLocator::NoSDKError
         sdk = @locator.latest_sdk
       end
       # Only return an SDK older than the OS version if it was specifically requested
diff --git a/Library/Homebrew/os/mac/sdk.rb b/Library/Homebrew/os/mac/sdk.rb
index a4eba815d2..5060b7807c 100644
--- a/Library/Homebrew/os/mac/sdk.rb
+++ b/Library/Homebrew/os/mac/sdk.rb
@@ -11,7 +11,7 @@ module OS
       end
     end
 
-    class SDKLocator
+    class BaseSDKLocator
       class NoSDKError < StandardError; end
 
       def sdk_for(v)
@@ -30,15 +30,12 @@ module OS
 
       private
 
+      def sdk_prefix
+        ""
+      end
+
       def sdk_paths
         @sdk_paths ||= begin
-          # Xcode.prefix is pretty smart, so let's look inside to find the sdk
-          sdk_prefix = "#{Xcode.prefix}/Platforms/MacOSX.platform/Developer/SDKs"
-          # Xcode < 4.3 style
-          sdk_prefix = "/Developer/SDKs" unless File.directory? sdk_prefix
-          # Finally query Xcode itself (this is slow, so check it last)
-          sdk_prefix = File.join(Utils.popen_read(DevelopmentTools.locate("xcrun"), "--show-sdk-platform-path").chomp, "Developer", "SDKs") unless File.directory? sdk_prefix
-
           # Bail out if there is no SDK prefix at all
           if !File.directory? sdk_prefix
             {}
@@ -55,5 +52,43 @@ module OS
         end
       end
     end
+
+    class XcodeSDKLocator < BaseSDKLocator
+      private
+
+      def sdk_prefix
+        @sdk_prefix ||= begin
+          # Xcode.prefix is pretty smart, so let's look inside to find the sdk
+          sdk_prefix = "#{Xcode.prefix}/Platforms/MacOSX.platform/Developer/SDKs"
+          # Xcode < 4.3 style
+          sdk_prefix = "/Developer/SDKs" unless File.directory? sdk_prefix
+          # Finally query Xcode itself (this is slow, so check it last)
+          sdk_prefix = File.join(Utils.popen_read(DevelopmentTools.locate("xcrun"), "--show-sdk-platform-path").chomp, "Developer", "SDKs") unless File.directory? sdk_prefix
+
+          sdk_prefix
+        end
+      end
+    end
+
+    class CLTSDKLocator < BaseSDKLocator
+      private
+
+      # While CLT SDKs existed prior to Xcode 10, those packages also
+      # installed a traditional Unix-style header layout and we prefer
+      # using that
+      # As of Xcode 10, the Unix-style headers are installed via a
+      # separate package, so we can't rely on their being present.
+      # This will only look up SDKs on Xcode 10 or newer, and still
+      # return nil SDKs for Xcode 9 and older.
+      def sdk_prefix
+        @sdk_prefix ||= begin
+          if !CLT.separate_header_package?
+            ""
+          else
+            "#{CLT::PKG_PATH}/SDKs"
+          end
+        end
+      end
+    end
   end
 end