diff --git a/chanfig/nested_dict.py b/chanfig/nested_dict.py
index c0459fcc..c613feba 100644
--- a/chanfig/nested_dict.py
+++ b/chanfig/nested_dict.py
@@ -557,6 +557,84 @@ def pop(self, name: Any, default: Any = Null) -> Any:
             raise KeyError(name)
         return super().pop(name)
 
+    def setdefault(
+        self,
+        name: Any,
+        value: Any,
+        convert_mapping: bool | None = None,
+    ) -> Any:
+        r"""
+        Set default value for `NestedDict`.
+
+        Args:
+            name:
+            value:
+            convert_mapping: Whether to convert `Mapping` to `NestedDict`.
+                Defaults to self.convert_mapping.
+
+        Returns:
+            value: If `NestedDict` does not contain `name`, return `value`.
+
+        Examples:
+            >>> d = NestedDict({"i.d": 1013, "f.n": "chang", "n.a.b.c": 1})
+            >>> d.setdefault("d.i", 1031)
+            1031
+            >>> d.setdefault("i.d", "chang")
+            1013
+            >>> d.setdefault("f.n", 1013)
+            'chang'
+            >>> d.setdefault("n.a.b.d", 2)
+            2
+        """
+        # pylint: disable=W0642
+
+        full_name = name
+        delimiter = self.getattr("delimiter", ".")
+        if convert_mapping is None:
+            convert_mapping = self.getattr("convert_mapping", False)
+        default_factory = self.getattr("default_factory", self.empty)
+        try:
+            while isinstance(name, str) and delimiter in name:
+                name, rest = name.split(delimiter, 1)
+                if name in dir(self) and isinstance(getattr(self.__class__, name), (property, cached_property)):
+                    self, name = getattr(self, name), rest
+                elif name not in self and isinstance(self, Mapping):
+                    default = (
+                        self.__missing__(name, default_factory()) if hasattr(self, "__missing__") else default_factory()
+                    )
+                    self, name = default, rest
+                else:
+                    self, name = self[name], rest
+                if isinstance(self, NestedDict):
+                    default_factory = self.getattr("default_factory", self.empty)
+        except (AttributeError, TypeError):
+            raise KeyError(name) from None
+
+        if isinstance(self, NestedDict) and name in self:
+            return super().get(name)
+        elif isinstance(self, Mapping) and name in self:
+            dict.__getitem__(self, name)
+
+        if (
+            convert_mapping
+            and isinstance(value, Mapping)
+            and not isinstance(value, default_factory if isinstance(default_factory, type) else type(self))
+            and not isinstance(value, Variable)
+        ):
+            try:
+                value = default_factory(**value)
+            except TypeError:
+                value = default_factory(value)
+        if isinstance(self, NestedDict):
+            super().set(name, value)
+        elif isinstance(self, Mapping):
+            dict.__setitem__(self, name, value)
+        else:
+            raise ValueError(
+                f"Cannot set `{full_name}` to `{value}`, as `{delimiter.join(full_name.split(delimiter)[:-1])}={self}`."
+            )
+        return value
+
     def validate(self) -> None:
         r"""
         Validate `NestedDict`.