diff --git a/src/click/core.py b/src/click/core.py
index 6adc65ccd..30295cdde 100644
--- a/src/click/core.py
+++ b/src/click/core.py
@@ -702,9 +702,29 @@ def lookup_default(self, name: str, call: bool = True) -> t.Any | None:
         :param call: If the default is a callable, call it. Disable to
             return the callable instead.
 
+        .. versionchanged:: 8.3.2
+            Returns ``None`` instead of internal sentinel when no default
+            is found in the default map.
+
         .. versionchanged:: 8.0
             Added the ``call`` parameter.
         """
+        value = self._lookup_default(name, call)
+        # Normalize UNSET to None for public API compatibility.
+        # Internal code uses _lookup_default directly to distinguish
+        # between "no value" (UNSET) and "explicitly set to None".
+        if value is UNSET:
+            return None
+        return value
+
+    def _lookup_default(self, name: str, call: bool = True) -> t.Any:
+        """Internal method to get default from default_map.
+
+        Returns UNSET sentinel if no default is found. Use lookup_default()
+        for public API which normalizes UNSET to None.
+
+        :meta private:
+        """
         if self.default_map is not None:
             value = self.default_map.get(name, UNSET)
 
@@ -2278,7 +2298,7 @@ def get_default(
         .. versionchanged:: 8.0
             Added the ``call`` parameter.
         """
-        value = ctx.lookup_default(self.name, call=False)  # type: ignore
+        value = ctx._lookup_default(self.name, call=False)  # type: ignore
 
         if value is UNSET:
             value = self.default
@@ -2321,7 +2341,7 @@ def consume_value(
                 source = ParameterSource.ENVIRONMENT
 
         if value is UNSET:
-            default_map_value = ctx.lookup_default(self.name)  # type: ignore
+            default_map_value = ctx._lookup_default(self.name)  # type: ignore
             if default_map_value is not UNSET:
                 value = default_map_value
                 source = ParameterSource.DEFAULT_MAP
diff --git a/tests/test_defaults.py b/tests/test_defaults.py
index 31eee844b..1b4a49314 100644
--- a/tests/test_defaults.py
+++ b/tests/test_defaults.py
@@ -110,3 +110,59 @@ def prefers_red(color):
     assert "red" in result.output
     result = runner.invoke(prefers_red, ["--green"])
     assert "green" in result.output
+
+
+def test_lookup_default_returns_none_not_sentinel(runner):
+    """Test that lookup_default returns None when parameter not in default_map.
+
+    Regression test for issue #3145.
+    """
+
+    @click.command()
+    @click.option("--email")
+    @click.pass_context
+    def cmd(ctx, email):
+        # When key not in default_map, lookup_default should return None
+        result = ctx.lookup_default("nonexistent")
+        assert result is None, f"Expected None, got {result!r}"
+        click.echo("OK")
+
+    result = runner.invoke(cmd)
+    assert result.exit_code == 0
+    assert "OK" in result.output
+
+
+def test_lookup_default_returns_none_with_empty_default_map(runner):
+    """Test that lookup_default returns None even when default_map exists but key missing."""
+
+    @click.command()
+    @click.option("--name", default="test")
+    @click.pass_context
+    def cmd(ctx, name):
+        # Set default_map but query for nonexistent key
+        ctx.default_map = {"other_param": "value"}
+        result = ctx.lookup_default("missing_key")
+        assert result is None, f"Expected None, got {result!r}"
+        click.echo("OK")
+
+    result = runner.invoke(cmd)
+    assert result.exit_code == 0
+    assert "OK" in result.output
+
+
+def test_lookup_default_still_returns_actual_defaults(runner):
+    """Test that lookup_default still returns actual values from default_map."""
+
+    @click.command()
+    @click.option("--name")
+    @click.pass_context
+    def cmd(ctx, name):
+        ctx.default_map = {"email": "test@example.com"}
+        # Should return the actual default when present
+        result = ctx.lookup_default("email")
+        assert result == "test@example.com"
+        click.echo("OK")
+
+    result = runner.invoke(cmd)
+    assert result.exit_code == 0
+    assert "OK" in result.output