diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 4149a45566..87d1217269 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   `ActiveSupport::Inflector.camelize` behaves expected when provided a symbol `:upper` or `:lower` argument. Matches
+    `String#camelize` behavior.
+
+    *Alex Ghiculescu*
+
 *   Raises an `ArgumentError` when the first argument of `ActiveSupport::Notification.subscribe` is
     invalid.
 
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 6747bd8f05..30aa321f2b 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -68,7 +68,9 @@ module ActiveSupport
     #   camelize(underscore('SSLError'))        # => "SslError"
     def camelize(term, uppercase_first_letter = true)
       string = term.to_s
-      if uppercase_first_letter
+      # String#camelize takes a symbol (:upper or :lower), so we match that to avoid surprises:
+      upcase = true == uppercase_first_letter || :upper == uppercase_first_letter
+      if upcase
         string = string.sub(/^[a-z\d]*/) { |match| inflections.acronyms[match] || match.capitalize! || match }
       else
         string = string.sub(inflections.acronyms_camelize_regex) { |match| match.downcase! || match }
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index cfde3a92bc..aef045e85e 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -112,6 +112,10 @@ class StringInflectionsTest < ActiveSupport::TestCase
     assert_equal("capital", "Capital".camelize(:lower))
   end
 
+  def test_camelize_upper
+    assert_equal("Capital", "Capital".camelize(:upper))
+  end
+
   def test_camelize_invalid_option
     e = assert_raise ArgumentError do
       "Capital".camelize(nil)
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index 4469f35595..84e400299d 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -140,10 +140,26 @@ class InflectorTest < ActiveSupport::TestCase
     end
   end
 
-  def test_camelize_with_lower_downcases_the_first_letter
+  def test_camelize_with_true_upcases_the_first_letter
+    assert_equal("Capital", ActiveSupport::Inflector.camelize("Capital", true))
+  end
+
+  def test_camelize_with_upper_upcases_the_first_letter
+    assert_equal("Capital", ActiveSupport::Inflector.camelize("Capital", :upper))
+  end
+
+  def test_camelize_with_false_downcases_the_first_letter
     assert_equal("capital", ActiveSupport::Inflector.camelize("Capital", false))
   end
 
+  def test_camelize_with_lower_downcases_the_first_letter
+    assert_equal("capital", ActiveSupport::Inflector.camelize("Capital", :lower))
+  end
+
+  def test_camelize_with_any_other_arg_downcases_the_first_letter
+    assert_equal("capital", ActiveSupport::Inflector.camelize("Capital", 222))
+  end
+
   def test_camelize_with_underscores
     assert_equal("CamelCase", ActiveSupport::Inflector.camelize("Camel_Case"))
   end