diff --git a/apistar/types.py b/apistar/types.py
index e96531da..b68b7082 100644
--- a/apistar/types.py
+++ b/apistar/types.py
@@ -52,6 +52,9 @@ def __new__(cls, name, bases, attrs):
 
 
 class Type(Mapping, metaclass=TypeMetaclass):
+
+    formatter = None
+
     def __init__(self, *args, **kwargs):
         definitions = None
         allow_coerce = False
@@ -116,8 +119,8 @@ def __getitem__(self, key):
         if value is None:
             return None
         validator = self.validator.properties[key]
-        if hasattr(validator, 'format') and validator.format in validators.FORMATS:
-            formatter = validators.FORMATS[validator.format]
+        if validator.formatter is not None:
+            formatter = validator.formatter
             return formatter.to_string(value)
         return value
 
diff --git a/apistar/validators.py b/apistar/validators.py
index 9df9e4f8..684cf56c 100644
--- a/apistar/validators.py
+++ b/apistar/validators.py
@@ -26,7 +26,8 @@ class Validator:
     errors = {}
     _creation_counter = 0
 
-    def __init__(self, title='', description='', default=NO_DEFAULT, allow_null=False, definitions=None, def_name=None):
+    def __init__(self, title='', description='', default=NO_DEFAULT, allow_null=False,
+                 definitions=None, def_name=None, formatter=None):
         definitions = {} if (definitions is None) else dict_type(definitions)
 
         assert isinstance(title, str)
@@ -46,6 +47,7 @@ def __init__(self, title='', description='', default=NO_DEFAULT, allow_null=Fals
         self.allow_null = allow_null
         self.definitions = definitions
         self.def_name = def_name
+        self.formatter = formatter
 
         # We need this global counter to determine what order fields have
         # been declared in when used with `Type`.
@@ -127,13 +129,15 @@ def __init__(self, max_length=None, min_length=None, pattern=None,
         self.pattern = pattern
         self.enum = enum
         self.format = format
+        if isinstance(self.format, str) and self.formatter is None and self.format in FORMATS:
+            self.formatter = FORMATS[self.format]
 
     def validate(self, value, definitions=None, allow_coerce=False):
         if value is None and self.allow_null:
             return None
         elif value is None:
             self.error('null')
-        elif self.format in FORMATS and FORMATS[self.format].is_native_type(value):
+        elif self.formatter is not None and self.formatter.is_native_type(value):
             return value
         elif not isinstance(value, str):
             self.error('type')
@@ -159,8 +163,8 @@ def validate(self, value, definitions=None, allow_coerce=False):
             if not re.search(self.pattern, value):
                 self.error('pattern')
 
-        if self.format in FORMATS:
-            return FORMATS[self.format].validate(value)
+        if self.formatter is not None:
+            return self.formatter.validate(value)
 
         return value
 
diff --git a/docs/api-guide/type-system.md b/docs/api-guide/type-system.md
index 3ec307b5..778fa4d9 100644
--- a/docs/api-guide/type-system.md
+++ b/docs/api-guide/type-system.md
@@ -270,3 +270,39 @@ You can also access the serialized string representation if needed.
 * `title` - A title to use in API schemas and documentation.
 * `description` - A description to use in API schemas and documentation.
 * `allow_null` - Indicates if `None` should be considered a valid value. Defaults to `False`.
+
+## Custom Formats
+
+Custom formatters can be provided for validators to enable them to return any native type
+
+```python
+from apistar.formats import BaseFormat
+
+class Foo:
+    def __init__(self, bar):
+        self.bar = bar
+
+class FooFormatter(BaseFormat):
+    def is_native_type(self, value):
+        return isinstance(value, Foo)
+
+    def to_string(self, value):
+        return value.bar
+
+    def validate(self, value):
+        if not isinstance(value, str) or not value.startswith('bar_'):
+            raise exceptions.ValidationError('Must start with bar_.')
+        return Foo(value)
+
+class Example(types.Type):
+    foo = validators.String(formatter=FooFormatter())
+    
+>>> data = {'foo': 'bar_foo'}
+>>> obj = Example(data)
+
+>>> obj.foo
+<__main__.Foo object at 0x7f143ec8ec88>
+
+>>> obj['foo']
+"bar_foo"
+```
\ No newline at end of file
diff --git a/tests/test_formats.py b/tests/test_formats.py
index b7fb0efc..5db504c0 100644
--- a/tests/test_formats.py
+++ b/tests/test_formats.py
@@ -3,6 +3,7 @@
 import pytest
 
 from apistar import exceptions, types, validators
+from apistar.formats import BaseFormat
 
 UTC = datetime.timezone.utc
 
@@ -120,3 +121,35 @@ class Example(types.Type):
     })
     assert example.when is None
     assert example['when'] is None
+
+
+def test_custom_formatter():
+    class Foo:
+        def __init__(self, bar):
+            self.bar = bar
+
+    class FooFormatter(BaseFormat):
+        def is_native_type(self, value):
+            return isinstance(value, Foo)
+
+        def to_string(self, value):
+            return value.bar
+
+        def validate(self, value):
+            if not isinstance(value, str) or not value.startswith('bar_'):
+                raise exceptions.ValidationError('Must start with bar_.')
+            return Foo(value)
+
+    class Example(types.Type):
+        foo = validators.String(formatter=FooFormatter())
+
+    with pytest.raises(exceptions.ValidationError) as exc:
+        example = Example({
+            'foo': 'foo'
+        })
+    assert exc.value.detail == {'foo': 'Must start with bar_.'}
+
+    example = Example({'foo': 'bar_foo'})
+    assert isinstance(example.foo, Foo)
+    assert example.foo.bar == 'bar_foo'
+    assert example['foo'] == 'bar_foo'