diff --git a/flang/runtime/CMakeLists.txt b/flang/runtime/CMakeLists.txt
--- a/flang/runtime/CMakeLists.txt
+++ b/flang/runtime/CMakeLists.txt
@@ -35,6 +35,7 @@
   allocatable.cpp
   assign.cpp
   buffer.cpp
+  command.cpp
   complex-reduction.c
   copy.cpp
   character.cpp
diff --git a/flang/runtime/command.cpp b/flang/runtime/command.cpp
new file mode 100644
--- /dev/null
+++ b/flang/runtime/command.cpp
@@ -0,0 +1,21 @@
+//===-- runtime/command.cpp -----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "command.h"
+#include "environment.h"
+
+namespace Fortran::runtime {
+CppTypeFor<TypeCategory::Integer, 4> RTNAME(ArgumentCount)() {
+  int argc{executionEnvironment.argc};
+  if (argc > 1) {
+    // C counts the command name as one of the arguments, but Fortran doesn't.
+    return argc - 1;
+  }
+  return 0;
+}
+} // namespace Fortran::runtime
diff --git a/flang/unittests/Runtime/CMakeLists.txt b/flang/unittests/Runtime/CMakeLists.txt
--- a/flang/unittests/Runtime/CMakeLists.txt
+++ b/flang/unittests/Runtime/CMakeLists.txt
@@ -1,6 +1,7 @@
 add_flang_unittest(FlangRuntimeTests
   BufferTest.cpp
   CharacterTest.cpp
+  CommandTest.cpp
   CrashHandlerFixture.cpp
   ExternalIOTest.cpp
   Format.cpp
diff --git a/flang/unittests/Runtime/CommandTest.cpp b/flang/unittests/Runtime/CommandTest.cpp
new file mode 100644
--- /dev/null
+++ b/flang/unittests/Runtime/CommandTest.cpp
@@ -0,0 +1,32 @@
+//===-- flang/unittests/RuntimeGTest/CommandTest.cpp ----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "../../runtime/command.h"
+#include "gtest/gtest.h"
+#include "../../runtime/descriptor.h"
+#include "../../runtime/main.h"
+
+using namespace Fortran::runtime;
+
+TEST(ArgumentCount, ZeroArguments) {
+  const char *argv[]{"aProgram"};
+  RTNAME(ProgramStart)(1, argv, {});
+  EXPECT_EQ(0, RTNAME(ArgumentCount)());
+}
+
+TEST(ArgumentCount, OneArgument) {
+  const char *argv[]{"aProgram", "anArgument"};
+  RTNAME(ProgramStart)(2, argv, {});
+  EXPECT_EQ(1, RTNAME(ArgumentCount)());
+}
+
+TEST(ArgumentCount, SeveralArguments) {
+  const char *argv[]{"aProgram", "arg1", "arg2", "arg3", "arg4"};
+  RTNAME(ProgramStart)(5, argv, {});
+  EXPECT_EQ(4, RTNAME(ArgumentCount)());
+}