diff --git a/crates/polars-core/src/datatypes/dtype.rs b/crates/polars-core/src/datatypes/dtype.rs
index 9eec5dff66f5..956d055a52c2 100644
--- a/crates/polars-core/src/datatypes/dtype.rs
+++ b/crates/polars-core/src/datatypes/dtype.rs
@@ -764,7 +764,6 @@ impl Display for DataType {
 }
 
 pub fn merge_dtypes(left: &DataType, right: &DataType) -> PolarsResult<DataType> {
-    // TODO! add struct
     use DataType::*;
     Ok(match (left, right) {
         #[cfg(feature = "dtype-categorical")]
@@ -794,6 +793,16 @@ pub fn merge_dtypes(left: &DataType, right: &DataType) -> PolarsResult<DataType>
             let merged = merge_dtypes(inner_l, inner_r)?;
             List(Box::new(merged))
         },
+        #[cfg(feature = "dtype-struct")]
+        (Struct(inner_l), Struct(inner_r)) => {
+            polars_ensure!(inner_l.len() == inner_r.len(), ComputeError: "cannot combine structs with differing amounts of fields ({} != {})", inner_l.len(), inner_r.len());
+            let fields = inner_l.iter().zip(inner_r.iter()).map(|(l, r)| {
+                polars_ensure!(l.name() == r.name(), ComputeError: "cannot combine structs with different fields ({} != {})", l.name(), r.name());
+                let merged = merge_dtypes(l.dtype(), r.dtype())?;
+                Ok(Field::new(l.name().clone(), merged))
+            }).collect::<PolarsResult<Vec<_>>>()?;
+            Struct(fields)
+        },
         #[cfg(feature = "dtype-array")]
         (Array(inner_l, width_l), Array(inner_r, width_r)) => {
             polars_ensure!(width_l == width_r, ComputeError: "widths of FixedSizeWidth Series are not equal");
diff --git a/py-polars/tests/unit/datatypes/test_categorical.py b/py-polars/tests/unit/datatypes/test_categorical.py
index b898c7c07999..c5888abee67e 100644
--- a/py-polars/tests/unit/datatypes/test_categorical.py
+++ b/py-polars/tests/unit/datatypes/test_categorical.py
@@ -845,3 +845,22 @@ def test_get_cat_categories_multiple_chunks() -> None:
     )
     df_cat = df.lazy().select(pl.col("e").cat.get_categories()).collect()
     assert len(df_cat) == 2
+
+
+@pytest.mark.parametrize(
+    "f",
+    [
+        lambda x: (pl.List(pl.Categorical), [x]),
+        lambda x: (pl.Struct({"a": pl.Categorical}), {"a": x}),
+    ],
+)
+def test_nested_categorical_concat(
+    f: Callable[[str], tuple[pl.DataType, list[str] | dict[str, str]]],
+) -> None:
+    dt, va = f("a")
+    _, vb = f("b")
+    a = pl.DataFrame({"x": [va]}, schema={"x": dt})
+    b = pl.DataFrame({"x": [vb]}, schema={"x": dt})
+
+    with pytest.raises(pl.exceptions.StringCacheMismatchError):
+        pl.concat([a, b])