diff --git a/CHANGES/1361.misc.rst b/CHANGES/1361.misc.rst
new file mode 100644
index 000000000..1f24b322b
--- /dev/null
+++ b/CHANGES/1361.misc.rst
@@ -0,0 +1 @@
+Made `istr` slotted in the Python implementation to prevent excessive creations of instance dictionaries -- by :user:`jonathandung4`.
diff --git a/multidict/_multidict_py.py b/multidict/_multidict_py.py
index e286713ec..6391b9274 100644
--- a/multidict/_multidict_py.py
+++ b/multidict/_multidict_py.py
@@ -34,8 +34,15 @@
 class istr(str):
     """Case insensitive str."""
 
+    __slots__ = ("__istr_identity__",)
+
     __is_istr__ = True
-    __istr_identity__: str | None = None
+    __istr_identity__: str | None
+
+    def __new__(cls, s: Any, /):
+        self = super().__new__(cls, s)
+        self.__istr_identity__ = None
+        return self
 
 
 _V = TypeVar("_V")
@@ -441,8 +448,7 @@ def _identity(self, key: str) -> str:
         if isinstance(key, istr):
             ret = key.__istr_identity__
             if ret is None:
-                ret = key.lower()
-                key.__istr_identity__ = ret
+                key.__istr_identity__ = ret = key.lower()
             return ret
         if isinstance(key, str):
             return key.lower()