diff --git a/flang/include/flang/Runtime/derived-api.h b/flang/include/flang/Runtime/derived-api.h
--- a/flang/include/flang/Runtime/derived-api.h
+++ b/flang/include/flang/Runtime/derived-api.h
@@ -38,6 +38,10 @@
 void RTNAME(Assign)(const Descriptor &, const Descriptor &,
     const char *sourceFile = nullptr, int sourceLine = 0);
 
+// Perform the test of the CLASS IS type guard statement of the SELECT TYPE
+// construct.
+bool RTNAME(ClassIs)(const Descriptor &, const typeInfo::DerivedType &);
+
 } // extern "C"
 } // namespace Fortran::runtime
 #endif // FORTRAN_RUNTIME_DERIVED_API_H_
diff --git a/flang/runtime/derived-api.cpp b/flang/runtime/derived-api.cpp
--- a/flang/runtime/derived-api.cpp
+++ b/flang/runtime/derived-api.cpp
@@ -39,6 +39,23 @@
   }
 }
 
+bool RTNAME(ClassIs)(
+    const Descriptor &descriptor, const typeInfo::DerivedType &derivedType) {
+  if (const DescriptorAddendum * addendum{descriptor.Addendum()}) {
+    if (const auto *derived{addendum->derivedType()}) {
+      if (derived == &derivedType)
+        return true;
+      const typeInfo::DerivedType *parent = derived->GetParentType();
+      while (parent) {
+        if (parent == &derivedType)
+          return true;
+        parent = parent->GetParentType();
+      }
+    }
+  }
+  return false;
+}
+
 // TODO: Assign()
 
 } // extern "C"