diff --git a/flang/include/flang/Optimizer/Builder/IntrinsicCall.h b/flang/include/flang/Optimizer/Builder/IntrinsicCall.h
index 7cb99d61a686e..ca15b4bc34b29 100644
--- a/flang/include/flang/Optimizer/Builder/IntrinsicCall.h
+++ b/flang/include/flang/Optimizer/Builder/IntrinsicCall.h
@@ -338,6 +338,7 @@ struct IntrinsicLibrary {
   mlir::Value genSign(mlir::Type, llvm::ArrayRef<mlir::Value>);
   mlir::Value genSind(mlir::Type, llvm::ArrayRef<mlir::Value>);
   fir::ExtendedValue genSize(mlir::Type, llvm::ArrayRef<fir::ExtendedValue>);
+  fir::ExtendedValue genSizeOf(mlir::Type, llvm::ArrayRef<fir::ExtendedValue>);
   mlir::Value genSpacing(mlir::Type resultType,
                          llvm::ArrayRef<mlir::Value> args);
   fir::ExtendedValue genSpread(mlir::Type, llvm::ArrayRef<fir::ExtendedValue>);
diff --git a/flang/lib/Optimizer/Builder/IntrinsicCall.cpp b/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
index 2f7ace658e475..ca5ab6fcea342 100644
--- a/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
+++ b/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
@@ -567,6 +567,10 @@ static constexpr IntrinsicHandler handlers[]{
        {"dim", asAddr, handleDynamicOptional},
        {"kind", asValue}}},
      /*isElemental=*/false},
+    {"sizeof",
+     &I::genSizeOf,
+     {{{"a", asBox}}},
+     /*isElemental=*/false},
     {"sleep", &I::genSleep, {{{"seconds", asValue}}}, /*isElemental=*/false},
     {"spacing", &I::genSpacing},
     {"spread",
@@ -5946,6 +5950,20 @@ IntrinsicLibrary::genSize(mlir::Type resultType,
       .getResults()[0];
 }
 
+// SIZEOF
+fir::ExtendedValue
+IntrinsicLibrary::genSizeOf(mlir::Type resultType,
+                            llvm::ArrayRef<fir::ExtendedValue> args) {
+  assert(args.size() == 1);
+  mlir::Value box = fir::getBase(args[0]);
+  mlir::Value eleSize = builder.create<fir::BoxEleSizeOp>(loc, resultType, box);
+  if (!fir::isArray(args[0]))
+    return eleSize;
+  mlir::Value arraySize = builder.createConvert(
+      loc, resultType, fir::runtime::genSize(builder, loc, box));
+  return builder.create<mlir::arith::MulIOp>(loc, eleSize, arraySize);
+}
+
 // TAND
 mlir::Value IntrinsicLibrary::genTand(mlir::Type resultType,
                                       llvm::ArrayRef<mlir::Value> args) {
diff --git a/flang/test/Lower/Intrinsics/sizeof.f90 b/flang/test/Lower/Intrinsics/sizeof.f90
new file mode 100644
index 0000000000000..959ca1692b514
--- /dev/null
+++ b/flang/test/Lower/Intrinsics/sizeof.f90
@@ -0,0 +1,23 @@
+! Test SIZEOF lowering for polymorphic entities.
+! RUN: bbc -emit-hlfir --polymorphic-type -o - %s | FileCheck %s
+
+integer(8) function test1(x)
+  class(*) :: x
+  test1 = sizeof(x)
+end function
+! CHECK-LABEL:   func.func @_QPtest1(
+! CHECK:           %[[VAL_3:.*]]:2 = hlfir.declare %{{.*}} {uniq_name = "_QFtest1Ex"} : (!fir.class<none>) -> (!fir.class<none>, !fir.class<none>)
+! CHECK:           %[[VAL_4:.*]] = fir.box_elesize %[[VAL_3]]#1 : (!fir.class<none>) -> i64
+! CHECK:           hlfir.assign %[[VAL_4]] to %{{.*}} : i64, !fir.ref<i64>
+
+integer(8) function test2(x)
+  class(*) :: x(:, :)
+  test2 = sizeof(x)
+end function
+! CHECK-LABEL:   func.func @_QPtest2(
+! CHECK:           %[[VAL_3:.*]]:2 = hlfir.declare %{{.*}} {uniq_name = "_QFtest2Ex"} : (!fir.class<!fir.array<?x?xnone>>) -> (!fir.class<!fir.array<?x?xnone>>, !fir.class<!fir.array<?x?xnone>>)
+! CHECK:           %[[VAL_4:.*]] = fir.box_elesize %[[VAL_3]]#1 : (!fir.class<!fir.array<?x?xnone>>) -> i64
+! CHECK:           %[[VAL_7:.*]] = fir.convert %[[VAL_3]]#1 : (!fir.class<!fir.array<?x?xnone>>) -> !fir.box<none>
+! CHECK:           %[[VAL_9:.*]] = fir.call @_FortranASize(%[[VAL_7]], %{{.*}}, %{{.*}}) fastmath<contract> : (!fir.box<none>, !fir.ref<i8>, i32) -> i64
+! CHECK:           %[[VAL_10:.*]] = arith.muli %[[VAL_4]], %[[VAL_9]] : i64
+! CHECK:           hlfir.assign %[[VAL_10]] to %{{.*}} : i64, !fir.ref<i64>