Index: flang/lib/Semantics/check-omp-structure.h
===================================================================
--- flang/lib/Semantics/check-omp-structure.h
+++ flang/lib/Semantics/check-omp-structure.h
@@ -76,6 +76,12 @@
 static OmpDirectiveSet nestedOrderedErrSet{Directive::OMPD_critical,
     Directive::OMPD_ordered, Directive::OMPD_atomic, Directive::OMPD_task,
     Directive::OMPD_taskloop};
+static OmpDirectiveSet nestedMasterErrSet{
+    OmpDirectiveSet{llvm::omp::Directive::OMPD_do,
+        llvm::omp::Directive::OMPD_sections, llvm::omp::Directive::OMPD_single,
+        llvm::omp::Directive::OMPD_workshare, llvm::omp::Directive::OMPD_atomic,
+        llvm::omp::Directive::OMPD_master} |
+    taskGeneratingSet};
 static OmpClauseSet privateSet{
     Clause::OMPC_private, Clause::OMPC_firstprivate, Clause::OMPC_lastprivate};
 static OmpClauseSet privateReductionSet{
@@ -200,6 +206,7 @@
   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 +366,12 @@
     HasInvalidWorksharingNesting(
         beginDir.source, {llvm::omp::Directive::OMPD_do});
   }
-  if (CurrentDirectiveIsNested())
+  if (CurrentDirectiveIsNested()) {
     CheckIfDoOrderedClause(beginDir);
+    if (GetContext().directive == llvm::omp::Directive::OMPD_master) {
+      CheckMasterNesting(x);
+    }
+  }
 
   CheckNoBranching(block, beginDir.v, beginDir.source);
 
@@ -381,6 +385,20 @@
   }
 }
 
+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 (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-nested-master.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/omp-nested-master.f90
@@ -0,0 +1,130 @@
+! 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 sections 
+  !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    do i = 1, 10
+      k = k + 1
+    end do
+    !$omp end master
+  !$omp end sections
+
+  !$omp single 
+  !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
+    !$omp master
+    do i = 1, 10
+      k = k + 1
+    end do
+    !$omp end master
+  !$omp end single
+
+
+
+  !$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
Index: flang/test/Semantics/omp-ordered-simd.f90
===================================================================
--- flang/test/Semantics/omp-ordered-simd.f90
+++ flang/test/Semantics/omp-ordered-simd.f90
@@ -118,6 +118,7 @@
 
   !$OMP TASK  
     C =  C - A * B
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
     !$OMP MASTER
     DO I = 1,N
       !ERROR: `ORDERED` region may not be closely nested inside of `CRITICAL`, `ORDERED`, explicit `TASK` or `TASKLOOP` region.
@@ -131,6 +132,7 @@
   !$OMP TASKLOOP
   DO J= 1,N  
     C =  C - A * B
+    !ERROR: `MASTER` region may not be closely nested inside of `WORKSHARING`, `LOOP`, `TASK`, `TASKLOOP`, or `ATOMIC` region.
     !$OMP MASTER
     DO I = 1,N
       !ERROR: `ORDERED` region may not be closely nested inside of `CRITICAL`, `ORDERED`, explicit `TASK` or `TASKLOOP` region.