diff --git a/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp b/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp
index fe806186ce25d..8b50ad8e583c5 100644
--- a/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp
+++ b/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp
@@ -502,10 +502,19 @@ ComputeRegionOp::wireHoistedValueThroughIns(Value value) {
 bool ComputeRegionOp::isEffectivelySerial() {
   auto *ctx = getContext();
 
+  // If there are no launch arguments, the compute region's parallelism
+  // has not yet been planned - and thus return NOT effectively serial.
+  if (getLaunchArgs().empty())
+    return false;
+
   if (getLaunchArg(GPUParallelDimAttr::seqDim(ctx)))
     return true;
 
   auto checkDim = [&](GPUParallelDimAttr dim) -> bool {
+    // Launch dimensions without an explicit `acc.par_width` for that dimension
+    // means that no such parallelism is assigned and thus defaults to width 1.
+    if (!getLaunchArg(dim))
+      return true;
     auto val = getKnownConstantLaunchArg(dim);
     return val && *val == 1;
   };
diff --git a/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp b/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp
index 44283c9091b4d..382e42f64fc40 100644
--- a/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp
+++ b/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp
@@ -93,11 +93,79 @@ class OpenACCCGOpsTest : public ::testing::Test {
     YieldOp::create(regionBuilder, loc);
   }
 
+  /// Build a single `acc.compute_region` with the given launch arguments.
+  ComputeRegionOp makeComputeRegion(HostContext &host, ValueRange launchArgs) {
+    Region sourceRegion;
+    populateSourceRegionSingleBlock(sourceRegion, context, loc, std::nullopt,
+                                    false);
+    IRMapping mapping;
+    // Setting an empty origin since it should not be relevant for the tests.
+    ComputeRegionOp cr = buildComputeRegion(
+        loc, launchArgs, {}, "", sourceRegion, host.rewriter, mapping);
+    return cr;
+  }
+
   MLIRContext context;
   OpBuilder b;
   Location loc;
 };
 
+//===----------------------------------------------------------------------===//
+// ComputeRegionOp::isEffectivelySerial
+//===----------------------------------------------------------------------===//
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialFalseWhenNoLaunchArgs) {
+  HostContext host(context, loc, b);
+  ComputeRegionOp cr = makeComputeRegion(host, {});
+  // No launch configuration yet - not treated as effectively serial.
+  EXPECT_FALSE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialSparseDimsConstantOne) {
+  HostContext host(context, loc, b);
+  Value c1 = arith::ConstantIndexOp::create(host.rewriter, loc, 1);
+  Value pwBx = ParWidthOp::create(host.rewriter, loc, c1,
+                                  GPUParallelDimAttr::blockXDim(&context));
+  Value pwTy = ParWidthOp::create(host.rewriter, loc, c1,
+                                  GPUParallelDimAttr::threadYDim(&context));
+  Value pwTx = ParWidthOp::create(host.rewriter, loc, c1,
+                                  GPUParallelDimAttr::threadXDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, {pwBx, pwTy, pwTx});
+  EXPECT_TRUE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest,
+       IsEffectivelySerialFalseWhenThreadXWidthGreaterThanOne) {
+  HostContext host(context, loc, b);
+  Value c2 = arith::ConstantIndexOp::create(host.rewriter, loc, 2);
+  Value pwTx = ParWidthOp::create(host.rewriter, loc, c2,
+                                  GPUParallelDimAttr::threadXDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, pwTx);
+  EXPECT_FALSE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialFalseWhenLaunchWidthUnknown) {
+  HostContext host(context, loc, b);
+  Value pwTx = ParWidthOp::create(host.rewriter, loc, Value(),
+                                  GPUParallelDimAttr::threadXDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, pwTx);
+  EXPECT_FALSE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialTrueWhenSeqLaunchPresent) {
+  HostContext host(context, loc, b);
+  Value c7 = arith::ConstantIndexOp::create(host.rewriter, loc, 7);
+  Value pwSeq = ParWidthOp::create(host.rewriter, loc, c7,
+                                   GPUParallelDimAttr::seqDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, pwSeq);
+  EXPECT_TRUE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
 //===----------------------------------------------------------------------===//
 // ComputeRegionOp::wireHoistedValueThroughIns
 //===----------------------------------------------------------------------===//