diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 3d708d91a7c4b..aab39d48ca2b5 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,14 @@
+*   `HashWithIndifferentAccess` better respects `#to_hash` on objects it's
+    given. In particular `#update`, `#merge`, `#replace` all accept objects
+    which respond to `#to_hash`, even if those objects are not Hashes directly.
+
+    Currently, if `HashWithIndifferentAccess.new` is given a non-Hash (even if
+    it responds to `#to_hash`) that object is treated as the default value,
+    rather than the initial keys and value. Changing that could break existing
+    code, so it will be updated in the next minor version.
+
+    *Peter Jaros*
+
 *   Added `Object#presence_in` to simplify value whitelisting.
 
     Before:
diff --git a/activesupport/lib/active_support/hash_with_indifferent_access.rb b/activesupport/lib/active_support/hash_with_indifferent_access.rb
index 594a4ca93824a..154f546943a69 100644
--- a/activesupport/lib/active_support/hash_with_indifferent_access.rb
+++ b/activesupport/lib/active_support/hash_with_indifferent_access.rb
@@ -72,6 +72,7 @@ def default(key = nil)
     end
 
     def self.new_from_hash_copying_default(hash)
+      hash = hash.to_hash
       new(hash).tap do |new_hash|
         new_hash.default = hash.default
       end
@@ -125,7 +126,7 @@ def update(other_hash)
       if other_hash.is_a? HashWithIndifferentAccess
         super(other_hash)
       else
-        other_hash.each_pair do |key, value|
+        other_hash.to_hash.each_pair do |key, value|
           if block_given? && key?(key)
             value = yield(convert_key(key), self[key], value)
           end
diff --git a/activesupport/test/core_ext/hash_ext_test.rb b/activesupport/test/core_ext/hash_ext_test.rb
index 40c8f03374154..697ec16db9025 100644
--- a/activesupport/test/core_ext/hash_ext_test.rb
+++ b/activesupport/test/core_ext/hash_ext_test.rb
@@ -23,6 +23,16 @@ def nested_under_indifferent_access
     end
   end
 
+  class HashByConversion
+    def initialize(hash)
+      @hash = hash
+    end
+
+    def to_hash
+      @hash
+    end
+  end
+
   def setup
     @strings = { 'a' => 1, 'b' => 2 }
     @nested_strings = { 'a' => { 'b' => { 'c' => 3 } } }
@@ -411,6 +421,12 @@ def test_indifferent_update
     assert [updated_with_strings, updated_with_symbols, updated_with_mixed].all? { |h| h.keys.size == 2 }
   end
 
+  def test_update_with_to_hash_conversion
+    hash = HashWithIndifferentAccess.new
+    hash.update HashByConversion.new({ :a => 1 })
+    assert_equal hash['a'], 1
+  end
+
   def test_indifferent_merging
     hash = HashWithIndifferentAccess.new
     hash[:a] = 'failure'
@@ -430,6 +446,12 @@ def test_indifferent_merging
     assert_equal 2, hash['b']
   end
 
+  def test_merge_with_to_hash_conversion
+    hash = HashWithIndifferentAccess.new
+    merged = hash.merge HashByConversion.new({ :a => 1 })
+    assert_equal merged['a'], 1
+  end
+
   def test_indifferent_replace
     hash = HashWithIndifferentAccess.new
     hash[:a] = 42
@@ -442,6 +464,18 @@ def test_indifferent_replace
     assert_same hash, replaced
   end
 
+  def test_replace_with_to_hash_conversion
+    hash = HashWithIndifferentAccess.new
+    hash[:a] = 42
+
+    replaced = hash.replace(HashByConversion.new(b: 12))
+
+    assert hash.key?('b')
+    assert !hash.key?(:a)
+    assert_equal 12, hash[:b]
+    assert_same hash, replaced
+  end
+
   def test_indifferent_merging_with_block
     hash = HashWithIndifferentAccess.new
     hash[:a] = 1