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
@@ -930,6 +930,19 @@
   }
 };
 
+/// Lower `fir.select_type` to LLVM IR dialect.
+struct SelectTypeOpConversion : public FIROpConversion<fir::SelectTypeOp> {
+  using FIROpConversion::FIROpConversion;
+
+  mlir::LogicalResult
+  matchAndRewrite(fir::SelectTypeOp select, OpAdaptor adaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    return rewriter.notifyMatchFailure(
+        select, "fir.select_type codegen is not implemented yet");
+    return failure();
+  }
+};
+
 /// conversion of fir::SelectRankOp to an if-then-else ladder
 struct SelectRankOpConversion : public FIROpConversion<fir::SelectRankOp> {
   using FIROpConversion::FIROpConversion;
@@ -1420,9 +1433,9 @@
         HasValueOpConversion, GlobalOpConversion, InsertOnRangeOpConversion,
         InsertValueOpConversion, IsPresentOpConversion, LoadOpConversion,
         NegcOpConversion, MulcOpConversion, SelectCaseOpConversion,
-        SelectOpConversion, SelectRankOpConversion, StoreOpConversion,
-        SubcOpConversion, UndefOpConversion, UnreachableOpConversion,
-        ZeroOpConversion>(typeConverter);
+        SelectOpConversion, SelectRankOpConversion, SelectTypeOpConversion,
+        StoreOpConversion, SubcOpConversion, UndefOpConversion,
+        UnreachableOpConversion, ZeroOpConversion>(typeConverter);
     mlir::populateStdToLLVMConversionPatterns(typeConverter, pattern);
     mlir::arith::populateArithmeticToLLVMConversionPatterns(typeConverter,
                                                             pattern);
diff --git a/flang/test/Fir/convert-to-llvm-invalid.fir b/flang/test/Fir/convert-to-llvm-invalid.fir
--- a/flang/test/Fir/convert-to-llvm-invalid.fir
+++ b/flang/test/Fir/convert-to-llvm-invalid.fir
@@ -50,3 +50,25 @@
 ^bb3:
   return
 }
+
+// -----
+
+// Test `fir.select_type` conversion failure. Not implemented yet.
+
+func @bar_select_type(%arg : !fir.box<!fir.type<derived3{f:f32}>>) -> i32 {
+  %0 = arith.constant 1 : i32
+  %2 = arith.constant 3 : i32
+
+  // expected-error@+1{{failed to legalize operation 'fir.select_type'}}
+  fir.select_type %arg : !fir.box<!fir.type<derived3{f:f32}>> [ 
+    #fir.instance<!fir.int<4>>,^bb1(%0:i32), 
+    #fir.instance<!fir.int<8>>,^bb2(%2:i32), 
+    unit,^bb5 ]
+^bb1(%a : i32) :
+  return %a : i32
+^bb2(%b : i32) :
+  return %b : i32
+^bb5 :
+  %zero = arith.constant 0 : i32
+  return %zero : i32
+}