diff --git a/src/click/core.py b/src/click/core.py
index f0a624be3..22f3b7a8d 100644
--- a/src/click/core.py
+++ b/src/click/core.py
@@ -685,6 +685,20 @@ def ensure_object(self, object_type: type[V]) -> V:
             self.obj = rv = object_type()
         return rv
 
+    def _default_map_has(self, name: str | None) -> bool:
+        """Check if :attr:`default_map` contains a real value for ``name``.
+
+        Returns ``False`` when the key is absent, the map is ``None``,
+        ``name`` is ``None``, or the stored value is the internal
+        :data:`UNSET` sentinel.
+        """
+        return (
+            name is not None
+            and self.default_map is not None
+            and name in self.default_map
+            and self.default_map[name] is not UNSET
+        )
+
     @t.overload
     def lookup_default(
         self, name: str, call: t.Literal[True] = True
@@ -705,15 +719,17 @@ def lookup_default(self, name: str, call: bool = True) -> t.Any | None:
         .. versionchanged:: 8.0
             Added the ``call`` parameter.
         """
-        if self.default_map is not None:
-            value = self.default_map.get(name)
+        if not self._default_map_has(name):
+            return None
 
-            if call and callable(value):
-                return value()
+        # Assert to make the type checker happy.
+        assert self.default_map is not None
+        value = self.default_map[name]
 
-            return value
+        if call and callable(value):
+            return value()
 
-        return None
+        return value
 
     def fail(self, message: str) -> t.NoReturn:
         """Aborts the execution of the program with a specific error
@@ -2281,9 +2297,7 @@ def get_default(
         name = self.name
         value = ctx.lookup_default(name, call=False) if name is not None else None
 
-        if value is None and not (
-            ctx.default_map is not None and name is not None and name in ctx.default_map
-        ):
+        if value is None and not ctx._default_map_has(name):
             value = self.default
 
         if call and callable(value):
@@ -2325,9 +2339,7 @@ def consume_value(
 
         if value is UNSET:
             default_map_value = ctx.lookup_default(self.name)  # type: ignore[arg-type]
-            if default_map_value is not None or (
-                ctx.default_map is not None and self.name in ctx.default_map
-            ):
+            if default_map_value is not None or ctx._default_map_has(self.name):
                 value = default_map_value
                 source = ParameterSource.DEFAULT_MAP
 
diff --git a/tests/test_defaults.py b/tests/test_defaults.py
index 7dc74f0b7..2226bda5e 100644
--- a/tests/test_defaults.py
+++ b/tests/test_defaults.py
@@ -2,6 +2,7 @@
 
 import click
 from click import UNPROCESSED
+from click._utils import UNSET
 
 
 @pytest.mark.parametrize(
@@ -354,3 +355,22 @@ def cli(value):
     result = runner.invoke(cli, args, **kwargs)
     assert result.exit_code == 0
     assert result.output == repr(expected)
+
+
+def test_unset_in_default_map(runner):
+    """An ``UNSET`` value in ``default_map`` should be treated as if
+    the key is absent, and so fallback to the parameter's own default.
+
+    Refs: https://github.com/pallets/click/pull/3224#issuecomment-3968643305
+    """
+
+    @click.command(
+        context_settings={"default_map": {"port": UNSET}},
+    )
+    @click.option("--port", default=8000)
+    def cli(port):
+        click.echo(f"port={port}")
+
+    result = runner.invoke(cli, [])
+    assert result.exit_code == 0
+    assert result.output.strip() == "port=8000"