diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 14a6c3c9f7..dfd9a07997 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Hashes can once again be passed to setters of `composed_of`, if all of the
+    mapping methods are methods implemented on `Hash`.
+
+    Fixes #25978.
+
+    *Sean Griffin*
+
 *   Fix the SELECT statement in `#table_comment` for MySQL.
 
     *Takeshi Akima*
diff --git a/activerecord/lib/active_record/aggregations.rb b/activerecord/lib/active_record/aggregations.rb
index 8bed5bca28..9f965052c5 100644
--- a/activerecord/lib/active_record/aggregations.rb
+++ b/activerecord/lib/active_record/aggregations.rb
@@ -261,8 +261,10 @@ module ActiveRecord
               part = converter.respond_to?(:call) ? converter.call(part) : klass.send(converter, part)
             end
 
-            if part.is_a?(Hash)
-              raise ArgumentError unless part.size == part.keys.max
+            hash_from_multiparameter_assignment = part.is_a?(Hash) &&
+              part.each_key.all? { |k| k.is_a?(Integer) }
+            if hash_from_multiparameter_assignment
+              raise ArgumentError unless part.size == part.each_key.max
               part = klass.new(*part.sort.map(&:last))
             end
 
diff --git a/activerecord/test/cases/aggregations_test.rb b/activerecord/test/cases/aggregations_test.rb
index 8a728902a8..4e865264c7 100644
--- a/activerecord/test/cases/aggregations_test.rb
+++ b/activerecord/test/cases/aggregations_test.rb
@@ -143,6 +143,11 @@ class AggregationsTest < ActiveRecord::TestCase
     customers(:barney).fullname = { first: "Barney", last: "Stinson" }
     assert_equal "Barney STINSON", customers(:barney).name
   end
+
+  def test_assigning_hash_without_custom_converter
+    customers(:barney).fullname_no_converter = { first: "Barney", last: "Stinson" }
+    assert_equal({ first: "Barney", last: "Stinson" }.to_s, customers(:barney).name)
+  end
 end
 
 class OverridingAggregationsTest < ActiveRecord::TestCase
diff --git a/activerecord/test/models/customer.rb b/activerecord/test/models/customer.rb
index 3338aaf7e1..d464759430 100644
--- a/activerecord/test/models/customer.rb
+++ b/activerecord/test/models/customer.rb
@@ -7,6 +7,7 @@ class Customer < ActiveRecord::Base
   composed_of :non_blank_gps_location, :class_name => "GpsLocation", :allow_nil => true, :mapping => %w(gps_location gps_location),
               :converter => lambda { |gps| self.gps_conversion_was_run = true; gps.blank? ? nil : GpsLocation.new(gps)}
   composed_of :fullname, :mapping => %w(name to_s), :constructor => Proc.new { |name| Fullname.parse(name) }, :converter => :parse
+  composed_of :fullname_no_converter, :mapping => %w(name to_s), class_name: "Fullname"
 end
 
 class Address