diff --git a/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb b/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb
index 6bd1e1bb66..f48a5158c3 100644
--- a/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb
+++ b/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb
@@ -20,11 +20,7 @@ module ActiveSupport
           end
 
           formatted_string =
-            if rounded_number.nan?
-              "NaN"
-            elsif rounded_number.infinite?
-              "Inf"
-            else
+            if rounded_number.finite?
               s = rounded_number.to_s("F")
               a, b = s.split(".", 2)
               if precision != 0
@@ -33,6 +29,9 @@ module ActiveSupport
                 a << b[0, precision]
               end
               a
+            else
+              # Infinity/NaN
+              "%f" % rounded_number
             end
         else
           formatted_string = rounded_number
diff --git a/activesupport/test/number_helper_test.rb b/activesupport/test/number_helper_test.rb
index ca9a02f536..9bdec994b7 100644
--- a/activesupport/test/number_helper_test.rb
+++ b/activesupport/test/number_helper_test.rb
@@ -208,6 +208,7 @@ module ActiveSupport
           assert_equal "9775." + "0" * 96, number_helper.number_to_rounded("9775", precision: 100, significant: true)
           assert_equal("97.7", number_helper.number_to_rounded(Rational(9772, 100), precision: 3, significant: true))
           assert_equal "28729870200000000000000", number_helper.number_to_rounded(0.287298702e23.to_d, precision: 0, significant: true)
+          assert_equal "-Inf", number_helper.number_to_rounded(-Float::INFINITY, precision: 0, significant: true)
         end
       end