diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -381,6 +381,9 @@
   let ObjectFormats = ["ELF"];
 }
 
+def TargetSupportsInitPriority : TargetSpec {
+  let CustomCode = [{ !Target.getTriple().isOSzOS() }];
+}
 // Attribute subject match rules that are used for #pragma clang attribute.
 //
 // A instance of AttrSubjectMatcherRule represents an individual match rule.
@@ -2221,7 +2224,7 @@
   let Documentation = [Undocumented];
 }
 
-def InitPriority : InheritableAttr {
+def InitPriority : InheritableAttr, TargetSpecificAttr<TargetSupportsInitPriority> {
   let Spellings = [GCC<"init_priority", /*AllowInC*/0>];
   let Args = [UnsignedArgument<"Priority">];
   let Subjects = SubjectList<[Var], ErrorDiag>;
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -79,7 +79,7 @@
 initialization being the opposite.
 
 This attribute is only supported for C++ and Objective-C++ and is ignored in
-other language modes.
+other language modes. Currently, this attribute is not implemented on z/OS.
   }];
 }
 
diff --git a/clang/test/SemaCXX/init-priority-attr.cpp b/clang/test/SemaCXX/init-priority-attr.cpp
--- a/clang/test/SemaCXX/init-priority-attr.cpp
+++ b/clang/test/SemaCXX/init-priority-attr.cpp
@@ -1,5 +1,7 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -fsyntax-only -DSYSTEM -verify %s
+// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fsyntax-only -DSYSTEM -verify %s
+// RUN: %clang_cc1 -triple=s390x-none-zos -fsyntax-only -verify=unknown %s
+// RUN: %clang_cc1 -triple=s390x-none-zos -fsyntax-only -DSYSTEM -verify=unknown-system %s
 
 #if defined(SYSTEM)
 #5 "init-priority-attr.cpp" 3 // system header
@@ -23,25 +25,35 @@
 extern Two koo[];
 
 Two foo __attribute__((init_priority(101))) ( 5, 6 );
+ // unknown-system-no-diagnostics
+ // unknown-warning@-2 {{unknown attribute 'init_priority' ignored}}
 
 Two goo __attribute__((init_priority(2,3))) ( 5, 6 ); // expected-error {{'init_priority' attribute takes one argument}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
 Two coo[2]  __attribute__((init_priority(100)));
 #if !defined(SYSTEM)
-// expected-error@-2 {{'init_priority' attribute requires integer constant between 101 and 65535 inclusive}}
+  // expected-error@-2 {{'init_priority' attribute requires integer constant between 101 and 65535 inclusive}}
+  // unknown-warning@-3 {{unknown attribute 'init_priority' ignored}}
 #endif
 
 Two boo[2]  __attribute__((init_priority(65536)));
 #if !defined(SYSTEM)
-// expected-error@-2 {{'init_priority' attribute requires integer constant between 101 and 65535 inclusive}}
+ // expected-error@-2 {{'init_priority' attribute requires integer constant between 101 and 65535 inclusive}}
+ // unknown-warning@-3 {{unknown attribute 'init_priority' ignored}}
 #endif
 
 Two koo[4]  __attribute__((init_priority(1.13))); // expected-error {{'init_priority' attribute requires an integer constant}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
 Two func()  __attribute__((init_priority(1001))); // expected-error {{'init_priority' attribute only applies to variables}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
+
 
 int i  __attribute__((init_priority(1001))); // expected-error {{can only use 'init_priority' attribute on file-scope definitions of objects of class type}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
 int main() {
-  Two foo __attribute__((init_priority(1001)));	// expected-error {{can only use 'init_priority' attribute on file-scope definitions of objects of class type}}
+  Two foo __attribute__((init_priority(1001))); // expected-error {{can only use 'init_priority' attribute on file-scope definitions of objects of class type}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 }
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1421,6 +1421,12 @@
 #define _LIBCPP_HAS_NO_FGETPOS_FSETPOS
 #endif
 
+#if __has_attribute(init_priority)
+# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
+#else
+# define _LIBCPP_INIT_PRIORITY_MAX
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/libcxx/src/experimental/memory_resource.cpp b/libcxx/src/experimental/memory_resource.cpp
--- a/libcxx/src/experimental/memory_resource.cpp
+++ b/libcxx/src/experimental/memory_resource.cpp
@@ -76,16 +76,6 @@
   ~ResourceInitHelper() {}
 };
 
-// Detect if the init_priority attribute is supported.
-#if (defined(_LIBCPP_COMPILER_GCC) && defined(__APPLE__)) \
-  || defined(_LIBCPP_COMPILER_MSVC)
-// GCC on Apple doesn't support the init priority attribute,
-// and MSVC doesn't support any GCC attributes.
-# define _LIBCPP_INIT_PRIORITY_MAX
-#else
-# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
-#endif
-
 // When compiled in C++14 this initialization should be a constant expression.
 // Only in C++11 is "init_priority" needed to ensure initialization order.
 #if _LIBCPP_STD_VER > 11
diff --git a/libcxx/src/iostream.cpp b/libcxx/src/iostream.cpp
--- a/libcxx/src/iostream.cpp
+++ b/libcxx/src/iostream.cpp
@@ -77,7 +77,7 @@
 #endif
 ;
 
-_LIBCPP_HIDDEN ios_base::Init __start_std_streams __attribute__((init_priority(101)));
+_LIBCPP_HIDDEN ios_base::Init __start_std_streams _LIBCPP_INIT_PRIORITY_MAX;
 
 // On Windows the TLS storage for locales needs to be initialized before we create
 // the standard streams, otherwise it may not be alive during program termination