diff --git a/flang/include/flang/Runtime/freestanding-tools.h b/flang/include/flang/Runtime/freestanding-tools.h
index 7f8d37d87e0e6..e94cb0a6c938c 100644
--- a/flang/include/flang/Runtime/freestanding-tools.h
+++ b/flang/include/flang/Runtime/freestanding-tools.h
@@ -12,6 +12,7 @@
 #include "flang/Common/api-attrs.h"
 #include "flang/Runtime/c-or-cpp.h"
 #include <algorithm>
+#include <cctype>
 #include <cstring>
 
 // The file defines a set of utilities/classes that might be
@@ -57,6 +58,11 @@
 #define STD_STRCMP_UNSUPPORTED 1
 #endif
 
+#if !defined(STD_TOUPPER_UNSUPPORTED) && \
+    (defined(__CUDACC__) || defined(__CUDA__)) && defined(__CUDA_ARCH__)
+#define STD_TOUPPER_UNSUPPORTED 1
+#endif
+
 namespace Fortran::runtime {
 
 #if STD_FILL_N_UNSUPPORTED
@@ -195,5 +201,18 @@ static inline RT_API_ATTRS int strcmp(const char *lhs, const char *rhs) {
 using std::strcmp;
 #endif // !STD_STRCMP_UNSUPPORTED
 
+#if STD_TOUPPER_UNSUPPORTED
+// Provides alternative implementation for std::toupper(), if
+// it is not supported.
+static inline RT_API_ATTRS int toupper(int ch) {
+  if (ch >= 'a' && ch <= 'z') {
+    return ch - 'a' + 'A';
+  }
+  return ch;
+}
+#else // !STD_TOUPPER_UNSUPPORTED
+using std::toupper;
+#endif // !STD_TOUPPER_UNSUPPORTED
+
 } // namespace Fortran::runtime
 #endif // FORTRAN_RUNTIME_FREESTANDING_TOOLS_H_
diff --git a/flang/lib/Decimal/decimal-to-binary.cpp b/flang/lib/Decimal/decimal-to-binary.cpp
index dc4aa82ac6fe4..eebd0736b67ad 100644
--- a/flang/lib/Decimal/decimal-to-binary.cpp
+++ b/flang/lib/Decimal/decimal-to-binary.cpp
@@ -11,9 +11,9 @@
 #include "flang/Common/leading-zero-bit-count.h"
 #include "flang/Decimal/binary-floating-point.h"
 #include "flang/Decimal/decimal.h"
+#include "flang/Runtime/freestanding-tools.h"
 #include <cinttypes>
 #include <cstring>
-#include <ctype.h>
 #include <utility>
 
 namespace Fortran::decimal {
@@ -468,8 +468,8 @@ BigRadixFloatingPointNumber<PREC, LOG10RADIX>::ConvertToBinary(
         ++q;
       }
     }
-    if ((!limit || limit >= q + 3) && toupper(q[0]) == 'N' &&
-        toupper(q[1]) == 'A' && toupper(q[2]) == 'N') {
+    if ((!limit || limit >= q + 3) && runtime::toupper(q[0]) == 'N' &&
+        runtime::toupper(q[1]) == 'A' && runtime::toupper(q[2]) == 'N') {
       // NaN
       p = q + 3;
       bool isQuiet{true};
@@ -493,11 +493,11 @@ BigRadixFloatingPointNumber<PREC, LOG10RADIX>::ConvertToBinary(
       }
       return {Real{NaN(isQuiet)}};
     } else { // Inf?
-      if ((!limit || limit >= q + 3) && toupper(q[0]) == 'I' &&
-          toupper(q[1]) == 'N' && toupper(q[2]) == 'F') {
-        if ((!limit || limit >= q + 8) && toupper(q[3]) == 'I' &&
-            toupper(q[4]) == 'N' && toupper(q[5]) == 'I' &&
-            toupper(q[6]) == 'T' && toupper(q[7]) == 'Y') {
+      if ((!limit || limit >= q + 3) && runtime::toupper(q[0]) == 'I' &&
+          runtime::toupper(q[1]) == 'N' && runtime::toupper(q[2]) == 'F') {
+        if ((!limit || limit >= q + 8) && runtime::toupper(q[3]) == 'I' &&
+            runtime::toupper(q[4]) == 'N' && runtime::toupper(q[5]) == 'I' &&
+            runtime::toupper(q[6]) == 'T' && runtime::toupper(q[7]) == 'Y') {
           p = q + 8;
         } else {
           p = q + 3;