diff --git a/xarray/core/indexes.py b/xarray/core/indexes.py
index 2242e57e482..33e974fec50 100644
--- a/xarray/core/indexes.py
+++ b/xarray/core/indexes.py
@@ -26,6 +26,7 @@
     is_allowed_extension_array_dtype,
     is_dict_like,
     is_scalar,
+    is_valid_numpy_dtype,
 )
 
 if TYPE_CHECKING:
@@ -610,7 +611,7 @@ def _asarray_tuplesafe(values):
 
 def _is_nested_tuple(possible_tuple):
     return isinstance(possible_tuple, tuple) and any(
-        isinstance(value, tuple | list | slice) for value in possible_tuple
+        isinstance(value, list | slice) for value in possible_tuple
     )
 
 
@@ -784,8 +785,12 @@ def concat(
             indexes_coord_dtypes = {idx.coord_dtype for idx in indexes}
             if len(indexes_coord_dtypes) == 1:
                 coord_dtype = next(iter(indexes_coord_dtypes))
-            else:
+            # Check if all dtypes are valid numpy dtypes before using np.result_type
+            # (e.g., pandas StringDtype is not a valid numpy dtype, GH#11317)
+            elif all(is_valid_numpy_dtype(dt) for dt in indexes_coord_dtypes):
                 coord_dtype = np.result_type(*indexes_coord_dtypes)
+            else:
+                coord_dtype = np.dtype("O")
 
         return cls(new_pd_index, dim=dim, coord_dtype=coord_dtype)
 
@@ -914,7 +919,12 @@ def join(
             index = self.index.intersection(other.index)
         if is_allowed_extension_array_dtype(index.dtype):
             return type(self)(index, self.dim)
-        coord_dtype = np.result_type(self.coord_dtype, other.coord_dtype)
+        if is_valid_numpy_dtype(self.coord_dtype) and is_valid_numpy_dtype(
+            other.coord_dtype
+        ):
+            coord_dtype = np.result_type(self.coord_dtype, other.coord_dtype)
+        else:
+            coord_dtype = np.dtype("O")
         return type(self)(index, self.dim, coord_dtype=coord_dtype)
 
     def reindex_like(
diff --git a/xarray/tests/test_concat.py b/xarray/tests/test_concat.py
index bc98d72d50c..4ed0315dfca 100644
--- a/xarray/tests/test_concat.py
+++ b/xarray/tests/test_concat.py
@@ -1811,3 +1811,13 @@ def test_concat_different_dims_in_different_child(self):
         actual = concat([dt1, dt2], dim="x")
         expected = DataTree.from_dict(coords={"/first/x": [1, 3], "/second/x": [2, 4]})
         assert actual.identical(expected)
+
+
+def test_concat_string_dtype_from_pd_index():
+    # Regression test for GH#11317: concat fails due to StringDtype introduced by pd.Index
+    da = DataArray([0], dims=["dim_a"], coords=dict(dim_a=["a"]))
+    db = DataArray([0])
+    db2 = concat([db], pd.Index(["b"], name="dim_a"))
+    result = concat([da, db2], dim="dim_a")
+    assert result.sizes["dim_a"] == 2
+    assert list(result.coords["dim_a"].values) == ["a", "b"]
diff --git a/xarray/tests/test_indexes.py b/xarray/tests/test_indexes.py
index 94adcc3b935..0dbdaba5043 100644
--- a/xarray/tests/test_indexes.py
+++ b/xarray/tests/test_indexes.py
@@ -509,6 +509,28 @@ def test_sel(self) -> None:
         with pytest.raises(IndexError):
             index.sel({"x": (slice(None), 1, "no_level")})
 
+    def test_sel_nested_tuple_key(self) -> None:
+        """Test that tuple-valued MultiIndex levels can be selected with a single key.
+
+        Regression test for GH#11341: when a MultiIndex level contains tuples,
+        selecting with a nested tuple key ((1, 1), 2) should collapse the dimension
+        just like selecting with a non-nested tuple key (1, 2).
+        """
+        # Create a MultiIndex where the first level contains tuples
+        nested_level_0 = pd.Index(
+            [(1, 1), (1, 1), (2, 2), (3, 3)], name="a", tupleize_cols=False
+        )
+        nested_level_1 = pd.Index([1, 2, 10, 20], name="b")
+        nested_mi = pd.MultiIndex.from_arrays([nested_level_0, nested_level_1])
+
+        index = PandasMultiIndex(nested_mi, "index")
+
+        # Select with a nested tuple key - should return scalar indexer
+        actual = index.sel({"index": ((1, 1), 2)})
+        # pandas.get_loc returns an integer for exact match
+        expected_dim_indexers = {"index": 1}
+        assert actual.dim_indexers == expected_dim_indexers
+
     def test_join(self):
         midx = pd.MultiIndex.from_product([["a", "aa"], [1, 2]], names=("one", "two"))
         level_coords_dtype = {"one": "=U2", "two": "i"}