diff --git a/flang/lib/Optimizer/CodeGen/CodeGen.cpp b/flang/lib/Optimizer/CodeGen/CodeGen.cpp
--- a/flang/lib/Optimizer/CodeGen/CodeGen.cpp
+++ b/flang/lib/Optimizer/CodeGen/CodeGen.cpp
@@ -161,6 +161,18 @@
     return success();
   }
 };
+
+// convert to LLVM IR dialect `unreachable`
+struct UnreachableOpConversion : public FIROpConversion<fir::UnreachableOp> {
+  using FIROpConversion::FIROpConversion;
+
+  mlir::LogicalResult
+  matchAndRewrite(fir::UnreachableOp unreach, OpAdaptor adaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    rewriter.replaceOpWithNewOp<mlir::LLVM::UnreachableOp>(unreach);
+    return success();
+  }
+};
 } // namespace
 
 namespace {
@@ -180,7 +192,7 @@
     auto loc = mlir::UnknownLoc::get(context);
     mlir::OwningRewritePatternList pattern(context);
     pattern.insert<AddrOfOpConversion, HasValueOpConversion, GlobalOpConversion,
-                   UndefOpConversion>(typeConverter);
+                   UndefOpConversion, UnreachableOpConversion>(typeConverter);
     mlir::populateStdToLLVMConversionPatterns(typeConverter, pattern);
     mlir::arith::populateArithmeticToLLVMConversionPatterns(typeConverter,
                                                             pattern);
diff --git a/flang/test/Fir/convert-to-llvm.fir b/flang/test/Fir/convert-to-llvm.fir
--- a/flang/test/Fir/convert-to-llvm.fir
+++ b/flang/test/Fir/convert-to-llvm.fir
@@ -81,3 +81,15 @@
 // CHECK:   %[[CST:.*]] = llvm.mlir.constant(dense<1> : vector<32x32xi32>) : !llvm.array<32 x array<32 x i32>>
 // CHECK:   llvm.return %[[CST]] : !llvm.array<32 x array<32 x i32>>
 // CHECK: }
+
+// -----
+
+// Verify that fir.unreachable is transformed to llvm.unreachable
+
+// CHECK:  llvm.func @test_unreachable() {
+// CHECK-NEXT:    llvm.unreachable
+// CHECK-NTEX:  }
+
+func @test_unreachable() {
+  fir.unreachable
+}