diff --git a/flang/include/flang/Parser/dump-parse-tree.h b/flang/include/flang/Parser/dump-parse-tree.h
--- a/flang/include/flang/Parser/dump-parse-tree.h
+++ b/flang/include/flang/Parser/dump-parse-tree.h
@@ -548,7 +548,7 @@
   NODE(parser, OpenMPDeclareReductionConstruct)
   NODE(parser, OpenMPDeclareSimdConstruct)
   NODE(parser, OpenMPDeclareTargetConstruct)
-  NODE(parser, OmpFlushMemoryClause)
+  NODE(parser, OmpMemoryOrderClause)
   NODE(parser, OpenMPFlushConstruct)
   NODE(parser, OpenMPLoopConstruct)
   NODE(parser, OpenMPSimpleStandaloneConstruct)
diff --git a/flang/include/flang/Parser/parse-tree.h b/flang/include/flang/Parser/parse-tree.h
--- a/flang/include/flang/Parser/parse-tree.h
+++ b/flang/include/flang/Parser/parse-tree.h
@@ -3701,8 +3701,8 @@
 // memory-order-clause -> acq_rel
 //                        release
 //                        acquire
-struct OmpFlushMemoryClause {
-  WRAPPER_CLASS_BOILERPLATE(OmpFlushMemoryClause, llvm::omp::Clause);
+struct OmpMemoryOrderClause {
+  WRAPPER_CLASS_BOILERPLATE(OmpMemoryOrderClause, OmpClause);
   CharBlock source;
 };
 
@@ -3710,7 +3710,7 @@
 struct OpenMPFlushConstruct {
   TUPLE_CLASS_BOILERPLATE(OpenMPFlushConstruct);
   CharBlock source;
-  std::tuple<Verbatim, std::optional<OmpFlushMemoryClause>,
+  std::tuple<Verbatim, std::optional<OmpMemoryOrderClause>,
       std::optional<OmpObjectList>>
       t;
 };
diff --git a/flang/lib/Lower/OpenMP.cpp b/flang/lib/Lower/OpenMP.cpp
--- a/flang/lib/Lower/OpenMP.cpp
+++ b/flang/lib/Lower/OpenMP.cpp
@@ -109,9 +109,9 @@
                     std::get<std::optional<Fortran::parser::OmpObjectList>>(
                         flushConstruct.t))
               genObjectList(*ompObjectList, converter, operandRange);
-            if (std::get<std::optional<Fortran::parser::OmpFlushMemoryClause>>(
+            if (std::get<std::optional<Fortran::parser::OmpMemoryOrderClause>>(
                     flushConstruct.t))
-              TODO("Handle OmpFlushMemoryClause");
+              TODO("Handle OmpMemoryOrderClause");
             converter.getFirOpBuilder().create<mlir::omp::FlushOp>(
                 converter.getCurrentLocation(), operandRange);
           },
diff --git a/flang/lib/Parser/openmp-parsers.cpp b/flang/lib/Parser/openmp-parsers.cpp
--- a/flang/lib/Parser/openmp-parsers.cpp
+++ b/flang/lib/Parser/openmp-parsers.cpp
@@ -302,18 +302,22 @@
 TYPE_PARSER(sourced(construct<OpenMPCancelConstruct>(verbatim("CANCEL"_tok),
     Parser<OmpCancelType>{}, maybe("IF" >> parenthesized(scalarLogicalExpr)))))
 
-// 2.17.8 Flush construct [OpenMP 5.0]
-//        flush -> FLUSH [memory-order-clause] [(variable-name-list)]
-//        memory-order-clause -> acq_rel
+// 2.17.7 Atomtic construct/2.17.8 Flush construct [OpenMP 5.0]
+//        memory-order-clause ->
+//                               seq_cst
+//                               acq_rel
 //                               release
 //                               acquire
-TYPE_PARSER(sourced(construct<OmpFlushMemoryClause>(
-    "ACQ_REL" >> pure(llvm::omp::Clause::OMPC_acq_rel) ||
-    "RELEASE" >> pure(llvm::omp::Clause::OMPC_release) ||
-    "ACQUIRE" >> pure(llvm::omp::Clause::OMPC_acquire))))
+//                               relaxed
+TYPE_PARSER(sourced(construct<OmpMemoryOrderClause>(
+    sourced("SEQ_CST" >> construct<OmpClause>(construct<OmpClause::SeqCst>()) ||
+        "ACQ_REL" >> construct<OmpClause>(construct<OmpClause::AcqRel>()) ||
+        "RELEASE" >> construct<OmpClause>(construct<OmpClause::Release>()) ||
+        "ACQUIRE" >> construct<OmpClause>(construct<OmpClause::Acquire>()) ||
+        "RELAXED" >> construct<OmpClause>(construct<OmpClause::Relaxed>())))))
 
 TYPE_PARSER(sourced(construct<OpenMPFlushConstruct>(verbatim("FLUSH"_tok),
-    maybe(Parser<OmpFlushMemoryClause>{}),
+    maybe(Parser<OmpMemoryOrderClause>{}),
     maybe(parenthesized(Parser<OmpObjectList>{})))))
 
 // Simple Standalone Directives
diff --git a/flang/lib/Parser/unparse.cpp b/flang/lib/Parser/unparse.cpp
--- a/flang/lib/Parser/unparse.cpp
+++ b/flang/lib/Parser/unparse.cpp
@@ -2419,25 +2419,11 @@
     Put("\n");
     EndOpenMP();
   }
-  void Unparse(const OmpFlushMemoryClause &x) {
-    switch (x.v) {
-    case llvm::omp::Clause::OMPC_acq_rel:
-      Word("ACQ_REL ");
-      break;
-    case llvm::omp::Clause::OMPC_release:
-      Word("RELEASE ");
-      break;
-    case llvm::omp::Clause::OMPC_acquire:
-      Word("ACQUIRE ");
-      break;
-    default:
-      break;
-    }
-  }
+  void Unparse(const OmpMemoryOrderClause &x) { Walk(x.v); }
   void Unparse(const OpenMPFlushConstruct &x) {
     BeginOpenMP();
     Word("!$OMP FLUSH ");
-    Walk(std::get<std::optional<OmpFlushMemoryClause>>(x.t));
+    Walk(std::get<std::optional<OmpMemoryOrderClause>>(x.t));
     Walk(" (", std::get<std::optional<OmpObjectList>>(x.t), ")");
     Put("\n");
     EndOpenMP();
diff --git a/flang/lib/Semantics/check-omp-structure.h b/flang/lib/Semantics/check-omp-structure.h
--- a/flang/lib/Semantics/check-omp-structure.h
+++ b/flang/lib/Semantics/check-omp-structure.h
@@ -150,6 +150,12 @@
   void Enter(const parser::OmpClause::Uniform &);
   void Enter(const parser::OmpClause::UseDevicePtr &);
   void Enter(const parser::OmpClause::IsDevicePtr &);
+  // Memory-order-clause
+  void Enter(const parser::OmpClause::SeqCst &);
+  void Enter(const parser::OmpClause::AcqRel &);
+  void Enter(const parser::OmpClause::Release &);
+  void Enter(const parser::OmpClause::Acquire &);
+  void Enter(const parser::OmpClause::Relaxed &);
 
   void Enter(const parser::OmpAlignedClause &);
   void Enter(const parser::OmpAllocateClause &);
diff --git a/flang/lib/Semantics/check-omp-structure.cpp b/flang/lib/Semantics/check-omp-structure.cpp
--- a/flang/lib/Semantics/check-omp-structure.cpp
+++ b/flang/lib/Semantics/check-omp-structure.cpp
@@ -378,6 +378,11 @@
 CHECK_SIMPLE_CLAUSE(Uniform, OMPC_uniform)
 CHECK_SIMPLE_CLAUSE(Untied, OMPC_untied)
 CHECK_SIMPLE_CLAUSE(UseDevicePtr, OMPC_use_device_ptr)
+CHECK_SIMPLE_CLAUSE(AcqRel, OMPC_acq_rel)
+CHECK_SIMPLE_CLAUSE(Acquire, OMPC_acquire)
+CHECK_SIMPLE_CLAUSE(SeqCst, OMPC_seq_cst)
+CHECK_SIMPLE_CLAUSE(Release, OMPC_release)
+CHECK_SIMPLE_CLAUSE(Relaxed, OMPC_relaxed)
 
 CHECK_REQ_SCALAR_INT_CLAUSE(Grainsize, OMPC_grainsize)
 CHECK_REQ_SCALAR_INT_CLAUSE(NumTasks, OMPC_num_tasks)
diff --git a/flang/test/Semantics/omp-clause-validity01.f90 b/flang/test/Semantics/omp-clause-validity01.f90
--- a/flang/test/Semantics/omp-clause-validity01.f90
+++ b/flang/test/Semantics/omp-clause-validity01.f90
@@ -62,7 +62,7 @@
      a = 3.14
   enddo
   !$omp end parallel
-  
+
   !$omp task private(b) allocate(b)
   do i = 1, N
      z = 2
@@ -80,7 +80,7 @@
      z = 2
   end do
   !$omp end target
- 
+
   !ERROR: ALLOCATE clause is not allowed on the TARGET DATA directive
   !$omp target data map(from: b) allocate(b)
   do i = 1, N
@@ -488,6 +488,11 @@
   !$omp flush release
   !$omp flush acquire
   !$omp flush release (c)
+  !ERROR: SEQ_CST clause is not allowed on the FLUSH directive
+  !$omp flush seq_cst
+  !ERROR: RELAXED clause is not allowed on the FLUSH directive
+  !$omp flush relaxed
+
   !$omp cancel DO
   !$omp cancellation point parallel