Index: flang/lib/Semantics/check-omp-structure.h
===================================================================
--- flang/lib/Semantics/check-omp-structure.h
+++ flang/lib/Semantics/check-omp-structure.h
@@ -76,6 +76,9 @@
 static OmpDirectiveSet nestedOrderedErrSet{Directive::OMPD_critical,
     Directive::OMPD_ordered, Directive::OMPD_atomic, Directive::OMPD_task,
     Directive::OMPD_taskloop};
+static OmpDirectiveSet nestedMasterErrSet{
+    OmpDirectiveSet{Directive::OMPD_atomic} | taskGeneratingSet | parallelSet |
+    doSet};
 static OmpClauseSet privateSet{
     Clause::OMPC_private, Clause::OMPC_firstprivate, Clause::OMPC_lastprivate};
 static OmpClauseSet privateReductionSet{
@@ -200,6 +203,7 @@
   bool CheckIntrinsicOperator(
       const parser::DefinedOperator::IntrinsicOperator &);
   void CheckReductionTypeList(const parser::OmpClause::Reduction &);
+  void CheckMasterNesting(const parser::OpenMPBlockConstruct &x);
   void CheckReductionArraySection(const parser::OmpObjectList &ompObjectList);
   void CheckIntentInPointerAndDefinable(
       const parser::OmpObjectList &, const llvm::omp::Clause);
Index: flang/lib/Semantics/check-omp-structure.cpp
===================================================================
--- flang/lib/Semantics/check-omp-structure.cpp
+++ flang/lib/Semantics/check-omp-structure.cpp
@@ -366,8 +366,10 @@
     HasInvalidWorksharingNesting(
         beginDir.source, {llvm::omp::Directive::OMPD_do});
   }
-  if (CurrentDirectiveIsNested())
+  if (CurrentDirectiveIsNested()) {
     CheckIfDoOrderedClause(beginDir);
+    CheckMasterNesting(x);
+  }
 
   CheckNoBranching(block, beginDir.v, beginDir.source);
 
@@ -381,6 +383,22 @@
   }
 }
 
+void OmpStructureChecker::CheckMasterNesting(
+    const parser::OpenMPBlockConstruct &x) {
+  // A MASTER region may not be `closely nested` inside a worksharing, loop,
+  // task, taskloop, or atomic region.
+  // TODO:  Expand the check to include `LOOP` construct as well when it is
+  // supported.
+  if (GetContext().directive == llvm::omp::Directive::OMPD_master) {
+    if (IsCloselyNestedRegion(llvm::omp::nestedMasterErrSet)) {
+      context_.Say(parser::FindSourceLocation(x),
+          "`MASTER` region may not be closely nested inside of `WORKSHARING`, "
+          "`LOOP`, `TASK`, `TASKLOOP`,"
+          " or `ATOMIC` region."_err_en_US);
+    }
+  }
+}
+
 void OmpStructureChecker::CheckIfDoOrderedClause(
     const parser::OmpBlockDirective &blkDirective) {
   if (blkDirective.v == llvm::omp::OMPD_ordered) {
Index: flang/test/Semantics/omp-clause-validity01.f90
===================================================================
--- flang/test/Semantics/omp-clause-validity01.f90
+++ flang/test/Semantics/omp-clause-validity01.f90
@@ -461,12 +461,14 @@
 ! 2.13.1 master
 
   !$omp parallel
+  !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
   !$omp master
   a=3.14
   !$omp end master
   !$omp end parallel
 
   !$omp parallel
+  !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
   !ERROR: NUM_THREADS clause is not allowed on the MASTER directive
   !$omp master num_threads(4)
   a=3.14
Index: flang/test/Semantics/omp-flush02.f90
===================================================================
--- flang/test/Semantics/omp-flush02.f90
+++ flang/test/Semantics/omp-flush02.f90
@@ -78,6 +78,7 @@
 
   !$omp parallel num_threads(4)
     array = (/1, 2, 3, 4, 5, 6, 7, 8, 9, 10/)
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
     !$omp master
       !$omp flush (array)
     !$omp end master
Index: flang/test/Semantics/omp-nested-master.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/omp-nested-master.f90
@@ -0,0 +1,147 @@
+! RUN: %S/test_errors.sh %s %t %flang_fc1 -fopenmp
+! OpenMP Version 4.5
+! Various checks with the nesting of MASTER construct
+
+program omp_nest_master
+  integer i, k, j
+  k = 0;
+
+  !$omp do
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+
+  !$omp do simd
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+
+  !$omp parallel do
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+
+  !$omp parallel do simd
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+
+  !$omp parallel
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+  !$omp end parallel
+
+  !$omp task
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+  !$omp end task
+
+  !$omp taskloop
+  do i = 1, 10
+    k = k + 1
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+  !$omp end taskloop
+
+  !$omp critical
+  do i = 1, 10
+    k = k + 1
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+  !$omp end critical
+
+  !$omp ordered
+  do i = 1, 10
+    k = k + 1
+    !$omp master
+    j = j -1
+    !$omp end master
+  end do
+  !$omp end ordered
+
+  !$omp ordered
+  do i = 1, 10
+    !$omp distribute
+    do k =1, 10
+      print *, "hello"
+      !$omp master
+      j = j -1
+      !$omp end master
+    end do
+    !$omp end distribute
+  end do
+  !$omp end ordered
+
+  !$omp critical
+  do i = 1, 10
+    !$omp distribute
+    do k =1, 10
+      print *, "hello"
+      !$omp master
+      j = j -1
+      !$omp end master
+    end do
+    !$omp end distribute
+  end do
+  !$omp end critical
+
+  !$omp taskloop
+  do i = 1, 10
+    !$omp distribute
+    do k =1, 10
+      print *, "hello"
+      !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+      !$omp master
+      j = j -1
+      !$omp end master
+    end do
+    !$omp end distribute
+  end do
+  !$omp end taskloop
+
+  !$omp task
+  do i = 1, 10
+    !$omp distribute
+    do k =1, 10
+      print *, "hello"
+      !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+      !$omp master
+      j = j -1
+      !$omp end master
+    end do
+    !$omp end distribute
+  end do
+  !$omp end task
+
+end program omp_nest_master