diff --git a/starlette/datastructures.py b/starlette/datastructures.py
index c9ffd4df..22ee32ee 100644
--- a/starlette/datastructures.py
+++ b/starlette/datastructures.py
@@ -1,3 +1,4 @@
+import itertools
 import tempfile
 import typing
 from collections import namedtuple
@@ -290,6 +291,83 @@ class ImmutableMultiDict(typing.Mapping):
         return f"{self.__class__.__name__}({repr(items)})"
 
 
+class MultiDict(ImmutableMultiDict):
+    def __setitem__(self, key: typing.Any, value: typing.Any) -> None:
+        self.setlist(key, [value])
+
+    def __delitem__(self, key: typing.Any) -> None:
+        self._list = [(k, v) for k, v in self._list if k != key]
+        del self._dict[key]
+
+    def pop(self, key: typing.Any, default: typing.Any = None) -> typing.Any:
+        self._list = [(k, v) for k, v in self._list if k != key]
+        return self._dict.pop(key, default)
+
+    def popitem(self) -> typing.Tuple:
+        key, value = self._dict.popitem()
+        self._list = [(k, v) for k, v in self._list if k != key]
+        return key, value
+
+    def poplist(self, key: typing.Any) -> typing.List:
+        values = [v for k, v in self._list if k == key]
+        self.pop(key)
+        return values
+
+    def clear(self) -> None:
+        self._dict.clear()
+        self._list.clear()
+
+    def setdefault(self, key: typing.Any, default: typing.Any = None) -> typing.Any:
+        if key not in self:
+            self._dict[key] = default
+            self._list.append((key, default))
+
+        return self[key]
+
+    def setlist(self, key: typing.Any, values: typing.List) -> None:
+        self.pop(key, None)
+        if not values:
+            values = []
+        else:
+            self._dict[key] = values[-1]
+        self._list.extend(((key, value) for value in values))
+
+    def appendlist(self, key: typing.Any, value: typing.Any) -> None:
+        self._list.append((key, value))
+        self._dict[key] = value
+
+    def update(
+        self,
+        values: typing.Union[
+            "MultiDict",
+            typing.Mapping,
+            typing.List[typing.Tuple[typing.Any, typing.Any]],
+        ] = None,
+        **kwargs: typing.Any,
+    ) -> None:
+        if values is None:
+            items_ = []  # type: typing.List
+        elif hasattr(values, "multi_items"):
+            values = typing.cast(MultiDict, values)
+            items_ = list(values.multi_items())
+        elif hasattr(values, "items"):
+            values = typing.cast(typing.Mapping, values)
+            items_ = list(values.items())
+        else:
+            values = typing.cast(
+                typing.List[typing.Tuple[typing.Any, typing.Any]], values
+            )
+            items_ = values
+
+        keys = {k for k, _ in itertools.chain(items_, kwargs.items())}
+        self._list = [
+            *((k, v) for k, v in self._list if k not in keys),
+            *items_,
+            *list(kwargs.items()),
+        ]
+        self._dict.update(itertools.chain(items_, kwargs.items()))
+
+
 class QueryParams(ImmutableMultiDict):
     """
     An immutable multidict.
diff --git a/tests/test_datastructures.py b/tests/test_datastructures.py
index bd33909d..4a96efb8 100644
--- a/tests/test_datastructures.py
+++ b/tests/test_datastructures.py
@@ -6,6 +6,7 @@ from starlette.datastructures import (
     DatabaseURL,
     FormData,
     Headers,
+    MultiDict,
     MutableHeaders,
     QueryParams,
 )
@@ -224,3 +225,100 @@ def test_formdata():
     assert FormData(form) == form
     assert FormData({"a": "123", "b": "789"}) == FormData([("a", "123"), ("b", "789")])
     assert FormData({"a": "123", "b": "789"}) != {"a": "123", "b": "789"}
+
+
+def test_multidict():
+    q = MultiDict([("a", "123"), ("a", "456"), ("b", "789")])
+    assert "a" in q
+    assert "A" not in q
+    assert "c" not in q
+    assert q["a"] == "456"
+    assert q.get("a") == "456"
+    assert q.get("nope", default=None) is None
+    assert q.getlist("a") == ["123", "456"]
+    assert list(q.keys()) == ["a", "b"]
+    assert list(q.values()) == ["456", "789"]
+    assert list(q.items()) == [("a", "456"), ("b", "789")]
+    assert len(q) == 2
+    assert list(q) == ["a", "b"]
+    assert dict(q) == {"a": "456", "b": "789"}
+    assert str(q) == "MultiDict([('a', '123'), ('a', '456'), ('b', '789')])"
+    assert repr(q) == "MultiDict([('a', '123'), ('a', '456'), ('b', '789')])"
+    assert MultiDict({"a": "123", "b": "456"}) == MultiDict(
+        [("a", "123"), ("b", "456")]
+    )
+    assert MultiDict({"a": "123", "b": "456"}) == MultiDict(
+        [("a", "123"), ("b", "456")]
+    )
+    assert MultiDict({"a": "123", "b": "456"}) == MultiDict({"b": "456", "a": "123"})
+    assert MultiDict() == MultiDict({})
+    assert MultiDict({"a": "123", "b": "456"}) != "invalid"
+
+    q = MultiDict([("a", "123"), ("a", "456")])
+    assert MultiDict(q) == q
+
+    q = MultiDict([("a", "123"), ("a", "456")])
+    q["a"] = "789"
+    assert q["a"] == "789"
+    assert q.getlist("a") == ["789"]
+
+    q = MultiDict([("a", "123"), ("a", "456")])
+    del q["a"]
+    assert q.get("a") is None
+    assert repr(q) == "MultiDict([])"
+
+    q = MultiDict([("a", "123"), ("a", "456"), ("b", "789")])
+    assert q.pop("a") == "456"
+    assert q.get("a", None) is None
+    assert repr(q) == "MultiDict([('b', '789')])"
+
+    q = MultiDict([("a", "123"), ("a", "456"), ("b", "789")])
+    item = q.popitem()
+    assert q.get(item[0]) is None
+
+    q = MultiDict([("a", "123"), ("a", "456"), ("b", "789")])
+    assert q.poplist("a") == ["123", "456"]
+    assert q.get("a") is None
+    assert repr(q) == "MultiDict([('b', '789')])"
+
+    q = MultiDict([("a", "123"), ("a", "456"), ("b", "789")])
+    q.clear()
+    assert q.get("a") is None
+    assert repr(q) == "MultiDict([])"
+
+    q = MultiDict([("a", "123")])
+    q.setlist("a", ["456", "789"])
+    assert q.getlist("a") == ["456", "789"]
+    q.setlist("b", [])
+    assert q.get("b") is None
+
+    q = MultiDict([("a", "123")])
+    assert q.setdefault("a", "456") == "123"
+    assert q.getlist("a") == ["123"]
+    assert q.setdefault("b", "456") == "456"
+    assert q.getlist("b") == ["456"]
+    assert repr(q) == "MultiDict([('a', '123'), ('b', '456')])"
+
+    q = MultiDict([("a", "123")])
+    q.appendlist("a", "456")
+    assert q.getlist("a") == ["123", "456"]
+    assert repr(q) == "MultiDict([('a', '123'), ('a', '456')])"
+
+    q = MultiDict([("a", "123"), ("b", "456")])
+    q.update({"a": "789"})
+    assert q.getlist("a") == ["789"]
+    q == MultiDict([("a", "789"), ("b", "456")])
+
+    q = MultiDict([("a", "123"), ("b", "456")])
+    q.update(q)
+    assert repr(q) == "MultiDict([('a', '123'), ('b', '456')])"
+
+    q = MultiDict([("a", "123"), ("b", "456")])
+    q.update(None)
+    assert repr(q) == "MultiDict([('a', '123'), ('b', '456')])"
+
+    q = MultiDict([("a", "123"), ("a", "456")])
+    q.update([("a", "123")])
+    assert q.getlist("a") == ["123"]
+    q.update([("a", "456")], a="789", b="123")
+    assert q == MultiDict([("a", "456"), ("a", "789"), ("b", "123")])