diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 45c3cb0690..cb3e27838b 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Invalid values assigned to a JSON column are assumed to be `nil`.
+
+    Fixes #18629.
+
+    *Sean Griffin*
+
 *   Add `ActiveRecord::Base#accessed_fields`, which can be used to quickly
     discover which fields were read from a model when you are looking to only
     select the data you need from the database.
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid/json.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid/json.rb
index e12ddd9901..7dadc09a44 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid/json.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid/json.rb
@@ -11,7 +11,7 @@ module ActiveRecord
 
           def type_cast_from_database(value)
             if value.is_a?(::String)
-              ::ActiveSupport::JSON.decode(value)
+              ::ActiveSupport::JSON.decode(value) rescue nil
             else
               super
             end
diff --git a/activerecord/test/cases/adapters/postgresql/json_test.rb b/activerecord/test/cases/adapters/postgresql/json_test.rb
index 5f6cda1986..f5ae872483 100644
--- a/activerecord/test/cases/adapters/postgresql/json_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/json_test.rb
@@ -179,6 +179,14 @@ module PostgresqlJSONSharedTestCases
     assert_equal({ 'one' => 'two', 'three' => 'four' }, json.payload)
     assert_not json.changed?
   end
+
+  def test_assigning_invalid_json
+    json = JsonDataType.new
+
+    json.payload = 'foo'
+
+    assert_nil json.payload
+  end
 end
 
 class PostgresqlJSONTest < ActiveRecord::TestCase