diff --git a/serde/de.py b/serde/de.py
index fd96f99b..bd0d78ee 100644
--- a/serde/de.py
+++ b/serde/de.py
@@ -16,7 +16,7 @@
 from beartype.door import is_bearable
 from beartype.roar import BeartypeCallHintParamViolation
 from dataclasses import dataclass, is_dataclass
-from typing import overload, TypeVar, Generic, Any, Optional, Union, Literal
+from typing import overload, TypeVar, Generic, Any, Optional, Union, Literal, Iterator
 from typing_extensions import dataclass_transform
 
 from .compat import (
@@ -985,11 +985,31 @@ def literal(self, arg: DeField[Any]) -> str:
         )
 
     def default(self, arg: DeField[Any], code: str) -> str:
-        if arg.alias:
-            aliases = (f'"{s}"' for s in [arg.name, *arg.alias])
-            exists = f'_exists_by_aliases({arg.datavar}, [{",".join(aliases)}])'
+        """
+        Renders supplying default value during deserialization.
+        """
+
+        def get_aliased_fields(arg: Field[Any]) -> Iterator[str]:
+            return (f'"{s}"' for s in [arg.name, *arg.alias])
+
+        if arg.flatten:
+            # When a field has the `flatten` attribute, iterate over its dataclass fields.
+            # This ensures that the code checks keys in the data while considering aliases.
+            flattened = []
+            for subarg in defields(arg.type):
+                if subarg.alias:
+                    aliases = get_aliased_fields(subarg)
+                    flattened.append(f'_exists_by_aliases({arg.datavar}, [{",".join(aliases)}])')
+                else:
+                    flattened.append(f'"{subarg.name}" in {arg.datavar}')
+            exists = " and ".join(flattened)
         else:
-            exists = f'"{arg.conv_name()}" in {arg.datavar}'
+            if arg.alias:
+                aliases = get_aliased_fields(arg)
+                exists = f'_exists_by_aliases({arg.datavar}, [{",".join(aliases)}])'
+            else:
+                exists = f'"{arg.conv_name()}" in {arg.datavar}'
+
         if has_default(arg):
             return f'({code}) if {exists} else serde_scope.defaults["{arg.name}"]'
         elif has_default_factory(arg):
diff --git a/tests/test_flatten.py b/tests/test_flatten.py
index 85c0b390..7adf26d6 100644
--- a/tests/test_flatten.py
+++ b/tests/test_flatten.py
@@ -81,3 +81,37 @@ class Bar:
         @serde
         class Foo:
             bar: list[Bar] = field(flatten=True)
+
+
+def test_flatten_default() -> None:
+    @serde
+    class Bar:
+        c: float = field(default=0.0)
+        d: bool = field(default=False)
+
+    @serde
+    class Foo:
+        a: int
+        b: str = field(default="foo")
+        bar: Bar = field(flatten=True, default_factory=Bar)
+
+    f = Foo(a=10, b="b", bar=Bar(c=100.0, d=True))
+    assert from_json(Foo, to_json(f)) == f
+
+    assert from_json(Foo, '{"a": 20}') == Foo(20, "foo", Bar())
+
+
+def test_flatten_default_alias() -> None:
+    @serde
+    class Bar:
+        a: float = field(default=0.0, alias=["aa"])  # type: ignore
+        b: bool = field(default=False, alias=["bb"])  # type: ignore
+
+    @serde
+    class Foo:
+        bar: Bar = field(flatten=True, default_factory=Bar)
+
+    f = Foo(bar=Bar(100.0, True))
+    assert from_json(Foo, to_json(f)) == f
+
+    assert from_json(Foo, '{"aa": 20.0, "bb": false}') == Foo(Bar(20.0, False))