diff --git a/flang/lib/Semantics/check-acc-structure.cpp b/flang/lib/Semantics/check-acc-structure.cpp
--- a/flang/lib/Semantics/check-acc-structure.cpp
+++ b/flang/lib/Semantics/check-acc-structure.cpp
@@ -187,6 +187,17 @@
 void AccStructureChecker::Enter(const parser::OpenACCStandaloneConstruct &x) {
   const auto &standaloneDir{std::get<parser::AccStandaloneDirective>(x.t)};
   PushContextAndClauseSets(standaloneDir.source, standaloneDir.v);
+  if (standaloneDir.v == llvm::acc::Directive::ACCD_set) {
+    // Restriction - 2255 Two instances of the same clause may not appear on
+    // the same directive.
+    // Since these clauses are already in requiredClauses set, they cannot
+    // be also in allowedOnceClauses set. The double restriction is enforced
+    // here.
+    SetContextAllowedOnce(GetContext().allowedOnceClauses +
+        llvm::acc::Clause::ACCC_default_async +
+        llvm::acc::Clause::ACCC_device_num +
+        llvm::acc::Clause::ACCC_device_type);
+  }
 }
 
 void AccStructureChecker::Leave(const parser::OpenACCStandaloneConstruct &x) {
diff --git a/flang/test/Semantics/acc-clause-validity.f90 b/flang/test/Semantics/acc-clause-validity.f90
--- a/flang/test/Semantics/acc-clause-validity.f90
+++ b/flang/test/Semantics/acc-clause-validity.f90
@@ -14,6 +14,7 @@
 !   2.11 Parallel Loop
 !   2.11 Kernels Loop
 !   2.11 Serial Loop
+!   2.14.3 Set
 !   2.16.13 Wait
 
 program openacc_clause_validity
@@ -42,6 +43,31 @@
   !$acc init device_type(2, i, j)
   !$acc init device_num(i) device_type(i, j) if(ifCondition)
 
+  !ERROR: At least one of DEFAULT_ASYNC, DEVICE_NUM, DEVICE_TYPE clause must appear on the SET directive
+  !$acc set
+
+  !ERROR: At least one of DEFAULT_ASYNC, DEVICE_NUM, DEVICE_TYPE clause must appear on the SET directive
+  !$acc set if(.TRUE.)
+
+  !ERROR: At most one DEFAULT_ASYNC clause can appear on the SET directive
+  !$acc set default_async(2) default_async(1)
+
+  !ERROR: At most one DEFAULT_ASYNC clause can appear on the SET directive
+  !$acc set default_async(2) default_async(1)
+
+  !ERROR: At most one DEVICE_NUM clause can appear on the SET directive
+  !$acc set device_num(1) device_num(i)
+
+  !ERROR: At most one DEVICE_TYPE clause can appear on the SET directive
+  !$acc set device_type(i) device_type(2, i, j)
+
+  !$acc set default_async(2)
+  !$acc set default_async(i)
+  !$acc set device_num(1)
+  !$acc set device_num(i)
+  !$acc set device_type(i)
+  !$acc set device_type(2, i, j)
+
   !ERROR: At least one of ATTACH, COPYIN, CREATE clause must appear on the ENTER DATA directive
   !$acc enter data
 
diff --git a/llvm/include/llvm/Frontend/OpenACC/ACC.td b/llvm/include/llvm/Frontend/OpenACC/ACC.td
--- a/llvm/include/llvm/Frontend/OpenACC/ACC.td
+++ b/llvm/include/llvm/Frontend/OpenACC/ACC.td
@@ -415,9 +415,15 @@
 // 2.14.3
 def ACC_Set : Directive<"set"> {
   let allowedOnceClauses = [
+    VersionedClause<ACCC_DefaultAsync>,
+    VersionedClause<ACCC_DeviceNum>,
+    VersionedClause<ACCC_DeviceType>,
     VersionedClause<ACCC_If>
   ];
   let requiredClauses = [
+    // The three following clauses are also in allowedOnceClauses list due to
+    // restriction 2255 - Two instances of the same clause may not appear on the
+    // same directive.
     VersionedClause<ACCC_DefaultAsync>,
     VersionedClause<ACCC_DeviceNum>,
     VersionedClause<ACCC_DeviceType>