diff --git a/Modules/Core/Common/include/itkNumberToString.h b/Modules/Core/Common/include/itkNumberToString.h
index 82cfb5800a1..bf7c1cd685a 100644
--- a/Modules/Core/Common/include/itkNumberToString.h
+++ b/Modules/Core/Common/include/itkNumberToString.h
@@ -43,16 +43,16 @@ class ITK_TEMPLATE_EXPORT NumberToString
 {
 public:
   std::string
-  operator()(TValue val);
+  operator()(TValue val) const;
 };
 
 // declaration of specialization
 template <>
 ITKCommon_EXPORT std::string
-NumberToString<double>::operator()(double val);
+NumberToString<double>::operator()(double val) const;
 template <>
 ITKCommon_EXPORT std::string
-NumberToString<float>::operator()(float val);
+NumberToString<float>::operator()(float val) const;
 
 } // namespace itk
 
diff --git a/Modules/Core/Common/include/itkNumberToString.hxx b/Modules/Core/Common/include/itkNumberToString.hxx
index f935126257b..b0a59472cbe 100644
--- a/Modules/Core/Common/include/itkNumberToString.hxx
+++ b/Modules/Core/Common/include/itkNumberToString.hxx
@@ -28,7 +28,7 @@ namespace itk
 
 template <typename TValue>
 std::string
-NumberToString<TValue>::operator()(TValue val)
+NumberToString<TValue>::operator()(TValue val) const
 {
   std::ostringstream output;
   output << static_cast<typename NumericTraits<TValue>::PrintType>(val);
diff --git a/Modules/Core/Common/src/itkNumberToString.cxx b/Modules/Core/Common/src/itkNumberToString.cxx
index 36a15fdb5e2..a8a03bd6144 100644
--- a/Modules/Core/Common/src/itkNumberToString.cxx
+++ b/Modules/Core/Common/src/itkNumberToString.cxx
@@ -26,7 +26,7 @@ namespace itk
 
 template <>
 std::string
-NumberToString<double>::operator()(double val)
+NumberToString<double>::operator()(double val) const
 {
   char                                               buf[256];
   const double_conversion::DoubleToStringConverter & converter =
@@ -42,7 +42,7 @@ NumberToString<double>::operator()(double val)
 
 template <>
 std::string
-NumberToString<float>::operator()(float val)
+NumberToString<float>::operator()(float val) const
 {
   char                                               buf[256];
   const double_conversion::DoubleToStringConverter & converter =
diff --git a/Modules/Core/Common/test/CMakeLists.txt b/Modules/Core/Common/test/CMakeLists.txt
index 799b939f807..48cb56b61e5 100644
--- a/Modules/Core/Common/test/CMakeLists.txt
+++ b/Modules/Core/Common/test/CMakeLists.txt
@@ -617,6 +617,7 @@ set(ITKCommonGTests
       itkIndexRangeGTest.cxx
       itkMersenneTwisterRandomVariateGeneratorGTest.cxx
       itkNeighborhoodAllocatorGTest.cxx
+      itkNumberToStringGTest.cxx
       itkOptimizerParametersGTest.cxx
       itkPointGTest.cxx
       itkShapedImageNeighborhoodRangeGTest.cxx
diff --git a/Modules/Core/Common/test/itkNumberToStringGTest.cxx b/Modules/Core/Common/test/itkNumberToStringGTest.cxx
new file mode 100644
index 00000000000..1a59800a311
--- /dev/null
+++ b/Modules/Core/Common/test/itkNumberToStringGTest.cxx
@@ -0,0 +1,144 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+
+// First include the header file to be tested:
+#include "itkNumberToString.h"
+#include <gtest/gtest.h>
+
+namespace
+{
+
+template <typename TValue>
+void
+Test_all_digits()
+{
+  const itk::NumberToString<TValue> numberToString{};
+
+  for (auto i = 9; i >= 0; --i)
+  {
+    EXPECT_EQ(numberToString(i), std::to_string(i));
+  }
+}
+
+
+template <typename TValue>
+void
+Test_non_finite_special_floating_point_values()
+{
+  using NumericLimitsType = std::numeric_limits<TValue>;
+  const itk::NumberToString<TValue> numberToString{};
+
+  EXPECT_EQ(numberToString(NumericLimitsType::quiet_NaN()), "NaN");
+  EXPECT_EQ(numberToString(NumericLimitsType::infinity()), "Infinity");
+  EXPECT_EQ(numberToString(-NumericLimitsType::infinity()), "-Infinity");
+}
+
+
+template <typename TValue>
+void
+Test_round_trip_of_finite_numeric_limits()
+{
+  using NumericLimitsType = std::numeric_limits<TValue>;
+  const itk::NumberToString<TValue> numberToString{};
+
+  for (const TValue expectedValue : { NumericLimitsType::lowest(),
+                                      NumericLimitsType::epsilon(),
+                                      NumericLimitsType::round_error(),
+                                      NumericLimitsType::denorm_min(),
+                                      NumericLimitsType::min(),
+                                      NumericLimitsType::max() })
+  {
+    std::istringstream inputStream{ numberToString(expectedValue) };
+    EXPECT_FALSE(inputStream.eof());
+    TValue actualValue;
+    inputStream >> actualValue;
+    EXPECT_TRUE(inputStream.eof());
+    EXPECT_EQ(actualValue, expectedValue);
+  }
+}
+
+
+template <typename TValue>
+void
+Test_decimal_notation_supports_up_to_twentyone_digits()
+{
+  const itk::NumberToString<TValue> numberToString{};
+
+  for (std::int8_t exponent{ 20 }; exponent > 0; --exponent)
+  {
+    const auto power_of_ten = std::pow(TValue{ 10 }, static_cast<TValue>(exponent));
+
+    // Test +/- 10 ^ exponent
+    EXPECT_EQ(numberToString(power_of_ten), '1' + std::string(exponent, '0'));
+    EXPECT_EQ(numberToString(-power_of_ten), "-1" + std::string(exponent, '0'));
+  }
+
+  for (std::int8_t exponent{ -6 }; exponent < 0; ++exponent)
+  {
+    const auto power_of_ten = std::pow(TValue{ 10 }, static_cast<TValue>(exponent));
+
+    // Test +/- 10 ^ exponent
+    EXPECT_EQ(numberToString(power_of_ten), "0." + std::string(-1 - exponent, '0') + '1');
+    EXPECT_EQ(numberToString(-power_of_ten), "-0." + std::string(-1 - exponent, '0') + '1');
+  }
+}
+
+} // namespace
+
+
+// Tests NumberToString for 0 to 9.
+TEST(NumberToString, SupportsAllDigits)
+{
+  Test_all_digits<int>();
+  Test_all_digits<float>();
+  Test_all_digits<double>();
+}
+
+
+// Tests that the function object returns a unique string for both positive and negative zero.
+TEST(NumberToString, HasUniqueZeroString)
+{
+  const std::string expectedZeroString = "0";
+
+  EXPECT_EQ(itk::NumberToString<int>{}(0), expectedZeroString);
+  EXPECT_EQ(itk::NumberToString<float>{}(+0.0f), expectedZeroString);
+  EXPECT_EQ(itk::NumberToString<float>{}(-0.0f), expectedZeroString);
+  EXPECT_EQ(itk::NumberToString<double>{}(+0.0), expectedZeroString);
+  EXPECT_EQ(itk::NumberToString<double>{}(-0.0), expectedZeroString);
+}
+
+
+TEST(NumberToString, NonFiniteSpecialFloatingPointValues)
+{
+  Test_non_finite_special_floating_point_values<float>();
+  Test_non_finite_special_floating_point_values<double>();
+}
+
+
+TEST(NumberToString, RoundTripOfFiniteFloatingPointNumericLimits)
+{
+  Test_round_trip_of_finite_numeric_limits<float>();
+  Test_round_trip_of_finite_numeric_limits<double>();
+}
+
+
+TEST(NumberToString, DecimalNotationUpTo21Digits)
+{
+  Test_decimal_notation_supports_up_to_twentyone_digits<float>();
+  Test_decimal_notation_supports_up_to_twentyone_digits<double>();
+}