diff --git a/flang/include/flang/Parser/dump-parse-tree.h b/flang/include/flang/Parser/dump-parse-tree.h
index 63fddc424182b..150a07d1142f6 100644
--- a/flang/include/flang/Parser/dump-parse-tree.h
+++ b/flang/include/flang/Parser/dump-parse-tree.h
@@ -484,6 +484,8 @@ class ParseTreeDumper {
   NODE(parser, OmpAtomicCapture)
   NODE(OmpAtomicCapture, Stmt1)
   NODE(OmpAtomicCapture, Stmt2)
+  NODE(parser, OmpAtomicCompare)
+  NODE(parser, OmpAtomicCompareIfStmt)
   NODE(parser, OmpAtomicRead)
   NODE(parser, OmpAtomicUpdate)
   NODE(parser, OmpAtomicWrite)
diff --git a/flang/include/flang/Parser/parse-tree.h b/flang/include/flang/Parser/parse-tree.h
index 22b7f9acd1af5..b8ae734596128 100644
--- a/flang/include/flang/Parser/parse-tree.h
+++ b/flang/include/flang/Parser/parse-tree.h
@@ -4126,6 +4126,20 @@ struct OmpAtomicCapture {
       t;
 };
 
+struct OmpAtomicCompareIfStmt {
+  UNION_CLASS_BOILERPLATE(OmpAtomicCompareIfStmt);
+  std::variant<common::Indirection<IfStmt>, common::Indirection<IfConstruct>> u;
+};
+
+// ATOMIC COMPARE (OpenMP 5.1, OPenMP 5.2 spec: 15.8.4)
+struct OmpAtomicCompare {
+  TUPLE_CLASS_BOILERPLATE(OmpAtomicCompare);
+  CharBlock source;
+  std::tuple<OmpAtomicClauseList, Verbatim, OmpAtomicClauseList,
+      OmpAtomicCompareIfStmt, std::optional<OmpEndAtomic>>
+      t;
+};
+
 // ATOMIC
 struct OmpAtomic {
   TUPLE_CLASS_BOILERPLATE(OmpAtomic);
@@ -4138,11 +4152,11 @@ struct OmpAtomic {
 // 2.17.7 atomic ->
 //        ATOMIC [atomic-clause-list] atomic-construct [atomic-clause-list] |
 //        ATOMIC [atomic-clause-list]
-//        atomic-construct -> READ | WRITE | UPDATE | CAPTURE
+//        atomic-construct -> READ | WRITE | UPDATE | CAPTURE | COMPARE
 struct OpenMPAtomicConstruct {
   UNION_CLASS_BOILERPLATE(OpenMPAtomicConstruct);
   std::variant<OmpAtomicRead, OmpAtomicWrite, OmpAtomicCapture, OmpAtomicUpdate,
-      OmpAtomic>
+      OmpAtomicCompare, OmpAtomic>
       u;
 };
 
diff --git a/flang/lib/Lower/OpenMP/OpenMP.cpp b/flang/lib/Lower/OpenMP/OpenMP.cpp
index a2779213a1a15..80a17e97ee67f 100644
--- a/flang/lib/Lower/OpenMP/OpenMP.cpp
+++ b/flang/lib/Lower/OpenMP/OpenMP.cpp
@@ -2896,6 +2896,10 @@ static void genOMP(lower::AbstractConverter &converter, lower::SymMap &symTable,
                                           parser::OmpAtomicClauseList>(
                 converter, atomicCapture, loc);
           },
+          [&](const parser::OmpAtomicCompare &atomicCompare) {
+            mlir::Location loc = converter.genLocation(atomicCompare.source);
+            TODO(loc, "OpenMP atomic compare");
+          },
       },
       atomicConstruct.u);
 }
diff --git a/flang/lib/Parser/openmp-parsers.cpp b/flang/lib/Parser/openmp-parsers.cpp
index 630acf9a6b256..5295cbcff09bc 100644
--- a/flang/lib/Parser/openmp-parsers.cpp
+++ b/flang/lib/Parser/openmp-parsers.cpp
@@ -912,6 +912,17 @@ TYPE_PARSER("ATOMIC" >>
         Parser<OmpAtomicClauseList>{} / endOmpLine, statement(assignmentStmt),
         statement(assignmentStmt), Parser<OmpEndAtomic>{} / endOmpLine)))
 
+TYPE_PARSER(construct<OmpAtomicCompareIfStmt>(indirect(Parser<IfStmt>{})) ||
+    construct<OmpAtomicCompareIfStmt>(indirect(Parser<IfConstruct>{})))
+
+// OMP ATOMIC [MEMORY-ORDER-CLAUSE-LIST] COMPARE [MEMORY-ORDER-CLAUSE-LIST]
+TYPE_PARSER("ATOMIC" >>
+    sourced(construct<OmpAtomicCompare>(
+        Parser<OmpAtomicClauseList>{} / maybe(","_tok), verbatim("COMPARE"_tok),
+        Parser<OmpAtomicClauseList>{} / endOmpLine,
+        Parser<OmpAtomicCompareIfStmt>{},
+        maybe(Parser<OmpEndAtomic>{} / endOmpLine))))
+
 // OMP ATOMIC [MEMORY-ORDER-CLAUSE-LIST] UPDATE [MEMORY-ORDER-CLAUSE-LIST]
 TYPE_PARSER("ATOMIC" >>
     sourced(construct<OmpAtomicUpdate>(
@@ -934,6 +945,7 @@ TYPE_PARSER("ATOMIC" >>
 // Atomic Construct
 TYPE_PARSER(construct<OpenMPAtomicConstruct>(Parser<OmpAtomicRead>{}) ||
     construct<OpenMPAtomicConstruct>(Parser<OmpAtomicCapture>{}) ||
+    construct<OpenMPAtomicConstruct>(Parser<OmpAtomicCompare>{}) ||
     construct<OpenMPAtomicConstruct>(Parser<OmpAtomicWrite>{}) ||
     construct<OpenMPAtomicConstruct>(Parser<OmpAtomicUpdate>{}) ||
     construct<OpenMPAtomicConstruct>(Parser<OmpAtomic>{}))
diff --git a/flang/lib/Parser/unparse.cpp b/flang/lib/Parser/unparse.cpp
index 4d6aaceb69c18..1e97262d66981 100644
--- a/flang/lib/Parser/unparse.cpp
+++ b/flang/lib/Parser/unparse.cpp
@@ -2520,6 +2520,16 @@ class UnparseVisitor {
     Word("!$OMP END ATOMIC\n");
     EndOpenMP();
   }
+  void Unparse(const OmpAtomicCompare &x) {
+    BeginOpenMP();
+    Word("!$OMP ATOMIC");
+    Walk(std::get<0>(x.t));
+    Word(" COMPARE");
+    Walk(std::get<2>(x.t));
+    Put("\n");
+    EndOpenMP();
+    Walk(std::get<OmpAtomicCompareIfStmt>(x.t));
+  }
   void Unparse(const OmpAtomicRead &x) {
     BeginOpenMP();
     Word("!$OMP ATOMIC");
diff --git a/flang/lib/Semantics/check-omp-structure.cpp b/flang/lib/Semantics/check-omp-structure.cpp
index 9cac652216fcf..0cf9493695607 100644
--- a/flang/lib/Semantics/check-omp-structure.cpp
+++ b/flang/lib/Semantics/check-omp-structure.cpp
@@ -2408,6 +2408,23 @@ void OmpStructureChecker::CheckAtomicUpdateStmt(
   ErrIfAllocatableVariable(var);
 }
 
+void OmpStructureChecker::CheckAtomicCompareConstruct(
+    const parser::OmpAtomicCompare &atomicCompareConstruct) {
+
+  // TODO: Check that the if-stmt is `if (var == expr) var = new`
+  //       [with or without then/end-do]
+
+  unsigned version{context_.langOptions().OpenMPVersion};
+  if (version < 51) {
+    context_.Say(atomicCompareConstruct.source,
+        "%s construct not allowed in %s, %s"_err_en_US,
+        atomicCompareConstruct.source, ThisVersion(version), TryVersion(51));
+  }
+
+  // TODO: More work needed here. Some of the Update restrictions need to
+  // be added, but Update isn't the same either.
+}
+
 // TODO: Allow cond-update-stmt once compare clause is supported.
 void OmpStructureChecker::CheckAtomicCaptureConstruct(
     const parser::OmpAtomicCapture &atomicCaptureConstruct) {
@@ -2553,6 +2570,16 @@ void OmpStructureChecker::Enter(const parser::OpenMPAtomicConstruct &x) {
                 &std::get<0>(atomicCapture.t), &std::get<2>(atomicCapture.t));
             CheckAtomicCaptureConstruct(atomicCapture);
           },
+          [&](const parser::OmpAtomicCompare &atomicCompare) {
+            const auto &dir{std::get<parser::Verbatim>(atomicCompare.t)};
+            PushContextAndClauseSets(
+                dir.source, llvm::omp::Directive::OMPD_atomic);
+            CheckAtomicMemoryOrderClause(
+                &std::get<0>(atomicCompare.t), &std::get<2>(atomicCompare.t));
+            CheckHintClause<const parser::OmpAtomicClauseList>(
+                &std::get<0>(atomicCompare.t), &std::get<2>(atomicCompare.t));
+            CheckAtomicCompareConstruct(atomicCompare);
+          },
       },
       x.u);
 }
diff --git a/flang/lib/Semantics/check-omp-structure.h b/flang/lib/Semantics/check-omp-structure.h
index df21ebac0f6d7..6b4fb51c2d99c 100644
--- a/flang/lib/Semantics/check-omp-structure.h
+++ b/flang/lib/Semantics/check-omp-structure.h
@@ -213,6 +213,7 @@ class OmpStructureChecker
   void CheckAtomicCaptureStmt(const parser::AssignmentStmt &);
   void CheckAtomicWriteStmt(const parser::AssignmentStmt &);
   void CheckAtomicCaptureConstruct(const parser::OmpAtomicCapture &);
+  void CheckAtomicCompareConstruct(const parser::OmpAtomicCompare &);
   void CheckAtomicConstructStructure(const parser::OpenMPAtomicConstruct &);
   void CheckDistLinear(const parser::OpenMPLoopConstruct &x);
   void CheckSIMDNest(const parser::OpenMPConstruct &x);
diff --git a/flang/test/Lower/OpenMP/Todo/atomic-compare.f90 b/flang/test/Lower/OpenMP/Todo/atomic-compare.f90
new file mode 100644
index 0000000000000..88ec6fe910b9e
--- /dev/null
+++ b/flang/test/Lower/OpenMP/Todo/atomic-compare.f90
@@ -0,0 +1,11 @@
+! RUN: %not_todo_cmd %flang_fc1 -emit-fir -fopenmp -fopenmp-version=51 -o - %s 2>&1 | FileCheck %s
+
+! CHECK: not yet implemented: OpenMP atomic compare
+program p
+  integer :: x
+  logical :: r
+  !$omp atomic compare
+  if (x .eq. 0) then
+     x = 2
+  end if
+end program p
diff --git a/flang/test/Parser/OpenMP/atomic-compare.f90 b/flang/test/Parser/OpenMP/atomic-compare.f90
new file mode 100644
index 0000000000000..5cd02698ff482
--- /dev/null
+++ b/flang/test/Parser/OpenMP/atomic-compare.f90
@@ -0,0 +1,16 @@
+! RUN: not %flang_fc1  -fopenmp-version=51 -fopenmp %s 2>&1 | FileCheck %s
+! OpenMP version for documentation purposes only - it isn't used until Sema.
+! This is testing for Parser errors that bail out before Sema. 
+program main
+   implicit none
+   integer :: i, j = 10
+   logical :: r
+
+  !CHECK: error: expected OpenMP construct
+  !$omp atomic compare write
+  r =  i .eq. j + 1
+
+  !CHECK: error: expected end of line
+  !$omp atomic compare num_threads(4)
+  r = i .eq. j
+end program main
diff --git a/flang/test/Parser/OpenMP/atomic-unparse.f90 b/flang/test/Parser/OpenMP/atomic-unparse.f90
index f9d8ec5d5c681..64fa79fb1d1a2 100644
--- a/flang/test/Parser/OpenMP/atomic-unparse.f90
+++ b/flang/test/Parser/OpenMP/atomic-unparse.f90
@@ -3,6 +3,7 @@
 program main
    implicit none
    integer :: i, j = 10
+   integer :: k
 !READ
 !$omp atomic read
    i = j
@@ -121,6 +122,49 @@ program main
    i = j
 !$omp end atomic
 
+!COMPARE
+!$omp atomic compare
+   if (k == i) k = j
+!$omp atomic seq_cst compare
+   if (k == j) then
+      k = i
+   end if
+!$omp atomic compare seq_cst
+   if (k .eq. j) then
+      k = i
+   end if
+!$omp atomic release compare
+   if (i .eq. j) k = i
+!$omp atomic compare release
+   if (i .eq. j) then
+      i = k
+   end if
+!$omp atomic acq_rel compare
+   if (k .eq. j) then
+      j = i
+   end if
+!$omp atomic compare acq_rel
+   if (i .eq. j) then
+      i = k
+   end if
+!$omp atomic acquire compare
+   if (i .eq. j + 1) then
+      i = j
+   end if
+   
+!$omp atomic compare acquire
+   if (i .eq. j) then
+      i = k
+   end if
+!$omp atomic relaxed compare
+   if (i .eq. j) then
+      i = k
+   end if
+!$omp atomic compare relaxed
+   if (i .eq. k) then
+      i = j
+   end if
+
 !ATOMIC
 !$omp atomic
    i = j
@@ -205,6 +249,20 @@ end program main
 !CHECK: !$OMP ATOMIC CAPTURE RELAXED
 !CHECK: !$OMP END ATOMIC
 
+!COMPARE
+
+!CHECK: !$OMP ATOMIC COMPARE
+!CHECK: !$OMP ATOMIC SEQ_CST COMPARE
+!CHECK: !$OMP ATOMIC COMPARE SEQ_CST
+!CHECK: !$OMP ATOMIC RELEASE COMPARE
+!CHECK: !$OMP ATOMIC COMPARE RELEASE
+!CHECK: !$OMP ATOMIC ACQ_REL COMPARE
+!CHECK: !$OMP ATOMIC COMPARE ACQ_REL
+!CHECK: !$OMP ATOMIC ACQUIRE COMPARE
+!CHECK: !$OMP ATOMIC COMPARE ACQUIRE
+!CHECK: !$OMP ATOMIC RELAXED COMPARE
+!CHECK: !$OMP ATOMIC COMPARE RELAXED
+
 !ATOMIC
 !CHECK: !$OMP ATOMIC
 !CHECK: !$OMP ATOMIC SEQ_CST
diff --git a/flang/test/Semantics/OpenMP/atomic-compare.f90 b/flang/test/Semantics/OpenMP/atomic-compare.f90
new file mode 100644
index 0000000000000..85644ad909107
--- /dev/null
+++ b/flang/test/Semantics/OpenMP/atomic-compare.f90
@@ -0,0 +1,79 @@
+! REQUIRES: openmp_runtime
+! RUN: %python %S/../test_errors.py %s %flang_fc1 %openmp_flags -fopenmp-version=51
+  use omp_lib
+  implicit none
+  ! Check atomic compare. This combines elements from multiple other "atomic*.f90", as
+  ! to avoid having several files with just a few lines in them. atomic compare needs
+  ! higher openmp version than the others, so need a separate file.
+  
+
+  real a, b, c
+  a = 1.0
+  b = 2.0
+  c = 3.0
+  !$omp parallel num_threads(4)
+  ! First a few things that should compile without error.
+  !$omp atomic seq_cst, compare
+  if (b .eq. a) then
+     b = c
+  end if
+
+  !$omp atomic seq_cst compare
+  if (a .eq. b) a = c
+  !$omp end atomic
+
+  !$omp atomic compare acquire hint(OMP_LOCK_HINT_CONTENDED)
+  if (b .eq. a) b = c
+
+  !$omp atomic release hint(OMP_LOCK_HINT_UNCONTENDED) compare
+  if (b .eq. a) b = c
+
+  !$omp atomic compare seq_cst
+  if (b .eq. c) b = a
+
+  !$omp atomic hint(1) acq_rel compare
+  if (b .eq. a) b = c
+  !$omp end atomic
+
+  ! Check for error conditions:
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one SEQ_CST clause can appear on the COMPARE directive
+  !$omp atomic seq_cst seq_cst compare
+  if (b .eq. c) b = a
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one SEQ_CST clause can appear on the COMPARE directive
+  !$omp atomic compare seq_cst seq_cst
+  if (b .eq. c) b = a
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one SEQ_CST clause can appear on the COMPARE directive
+  !$omp atomic seq_cst compare seq_cst
+  if (b .eq. c) b = a
+
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one ACQUIRE clause can appear on the COMPARE directive
+  !$omp atomic acquire acquire compare
+  if (b .eq. c) b = a
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one ACQUIRE clause can appear on the COMPARE directive
+  !$omp atomic compare acquire acquire
+  if (b .eq. c) b = a
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one ACQUIRE clause can appear on the COMPARE directive
+  !$omp atomic acquire compare acquire
+  if (b .eq. c) b = a
+
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one RELAXED clause can appear on the COMPARE directive
+  !$omp atomic relaxed relaxed compare
+  if (b .eq. c) b = a
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one RELAXED clause can appear on the COMPARE directive
+  !$omp atomic compare relaxed relaxed
+  if (b .eq. c) b = a
+  !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
+  !ERROR: At most one RELAXED clause can appear on the COMPARE directive
+  !$omp atomic relaxed compare relaxed
+  if (b .eq. c) b = a
+
+  !$omp end parallel
+end
diff --git a/flang/test/Semantics/OpenMP/atomic.f90 b/flang/test/Semantics/OpenMP/atomic.f90
index 44f06b7460bf1..0e100871ea9b4 100644
--- a/flang/test/Semantics/OpenMP/atomic.f90
+++ b/flang/test/Semantics/OpenMP/atomic.f90
@@ -35,6 +35,7 @@
   a = a + 1
   !ERROR: expected 'UPDATE'
   !ERROR: expected 'WRITE'
+  !ERROR: expected 'COMPARE'
   !ERROR: expected 'CAPTURE'
   !ERROR: expected 'READ'
   !$omp atomic num_threads(4)
@@ -49,6 +50,7 @@
 
   !ERROR: expected 'UPDATE'
   !ERROR: expected 'WRITE'
+  !ERROR: expected 'COMPARE'
   !ERROR: expected 'CAPTURE'
   !ERROR: expected 'READ'
   !$omp atomic num_threads write