diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
index 6fac5e58b354..af26d99fdb86 100644
--- a/activesupport/lib/active_support/core_ext/string/output_safety.rb
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -109,9 +109,4 @@ def html_safe
   def html_safe?
     false
   end
-
-  def html_safe!
-    ActiveSupport::Deprecation.warn("Use html_safe with your strings instead of html_safe! See http://yehudakatz.com/2010/02/01/safebuffers-and-rails-3-0/ for the full story.", caller)
-    self
-  end
 end
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index af3ec1260f79..27c2c3b243c8 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -298,11 +298,8 @@ def to_s
     assert !@string.html_safe?
   end
 
-  test "A string can be marked safe using html_safe!" do
-    assert_deprecated do
-      @string.html_safe!
-      assert @string.html_safe?
-    end
+  test "Marking a string html_safe! doesn't work unless rails_xss is installed" do
+    assert_raise(NoMethodError) { @string.html_safe! }
   end
 
   test "A string can be marked safe" do
@@ -310,12 +307,6 @@ def to_s
     assert string.html_safe?
   end
 
-  test "Marking a string safe returns the string using html_safe!" do
-    assert_deprecated do
-      assert_equal @string, @string.html_safe!
-    end
-  end
-
   test "Marking a string safe returns the string" do
     assert_equal @string, @string.html_safe
   end
@@ -336,17 +327,6 @@ def to_s
     assert string.html_safe?
   end
 
-  test "Adding a safe string to another safe string returns a safe string using html_safe!" do
-    assert_deprecated do
-      @other_string = "other".html_safe!
-      @string.html_safe!
-      @combination = @other_string + @string
-
-      assert_equal "otherhello", @combination
-      assert @combination.html_safe?
-    end
-  end
-
   test "Adding a safe string to another safe string returns a safe string" do
     @other_string = "other".html_safe
     string = @string.html_safe
@@ -356,20 +336,6 @@ def to_s
     assert @combination.html_safe?
   end
 
-  test "Adding an unsafe string to a safe string returns an unsafe string using html_safe!" do
-    assert_deprecated do
-      @other_string = "other".html_safe!
-      @combination = @other_string + "<foo>"
-      @other_combination = @string + "<foo>"
-
-      assert_equal "other<foo>", @combination
-      assert_equal "hello<foo>", @other_combination
-
-      assert !@combination.html_safe?
-      assert !@other_combination.html_safe?
-    end
-  end
-
   test "Adding an unsafe string to a safe string escapes it and returns a safe string" do
     @other_string = "other".html_safe
     @combination = @other_string + "<foo>"
@@ -382,16 +348,6 @@ def to_s
     assert !@other_combination.html_safe?
   end
 
-  test "Concatting safe onto unsafe yields unsafe using html_safe!" do
-    assert_deprecated do
-      @other_string = "other"
-      @string.html_safe!
-
-      @other_string.concat(@string)
-      assert !@other_string.html_safe?
-    end
-  end
-
   test "Concatting safe onto unsafe yields unsafe" do
     @other_string = "other"
     string = @string.html_safe
@@ -400,15 +356,6 @@ def to_s
     assert !@other_string.html_safe?
   end
 
-  test "Concatting unsafe onto safe yields unsafe using html_safe!" do
-    assert_deprecated do
-      @other_string = "other".html_safe!
-      string = @other_string.concat("<foo>")
-      assert_equal "other<foo>", string
-      assert !string.html_safe?
-    end
-  end
-
   test "Concatting unsafe onto safe yields escaped safe" do
     @other_string = "other".html_safe
     string = @other_string.concat("<foo>")
@@ -416,16 +363,6 @@ def to_s
     assert string.html_safe?
   end
 
-  test "Concatting safe onto safe yields safe using html_safe!" do
-    assert_deprecated do
-      @other_string = "other".html_safe!
-      @string.html_safe!
-
-      @other_string.concat(@string)
-      assert @other_string.html_safe?
-    end
-  end
-
   test "Concatting safe onto safe yields safe" do
     @other_string = "other".html_safe
     string = @string.html_safe
@@ -434,16 +371,6 @@ def to_s
     assert @other_string.html_safe?
   end
 
-  test "Concatting safe onto unsafe with << yields unsafe using html_safe!" do
-    assert_deprecated do
-      @other_string = "other"
-      @string.html_safe!
-
-      @other_string << @string
-      assert !@other_string.html_safe?
-    end
-  end
-
   test "Concatting safe onto unsafe with << yields unsafe" do
     @other_string = "other"
     string = @string.html_safe
@@ -452,15 +379,6 @@ def to_s
     assert !@other_string.html_safe?
   end
 
-  test "Concatting unsafe onto safe with << yields unsafe using html_safe!" do
-    assert_deprecated do
-      @other_string = "other".html_safe!
-      string = @other_string << "<foo>"
-      assert_equal "other<foo>", string
-      assert !string.html_safe?
-    end
-  end
-
   test "Concatting unsafe onto safe with << yields escaped safe" do
     @other_string = "other".html_safe
     string = @other_string << "<foo>"
@@ -468,16 +386,6 @@ def to_s
     assert string.html_safe?
   end
 
-  test "Concatting safe onto safe with << yields safe using html_safe!" do
-    assert_deprecated do
-      @other_string = "other".html_safe!
-      @string.html_safe!
-
-      @other_string << @string
-      assert @other_string.html_safe?
-    end
-  end
-
   test "Concatting safe onto safe with << yields safe" do
     @other_string = "other".html_safe
     @string.html_safe
@@ -486,15 +394,6 @@ def to_s
     assert @other_string.html_safe?
   end
 
-  test "Concatting a fixnum to safe always yields safe using html_safe!" do
-    assert_deprecated do
-      @string.html_safe!
-      @string.concat(13)
-      assert_equal "hello".concat(13), @string
-      assert @string.html_safe?
-    end
-  end
-
   test "Concatting a fixnum to safe always yields safe" do
     string = @string.html_safe
     string = string.concat(13)