diff --git a/ibis/backends/polars/compiler.py b/ibis/backends/polars/compiler.py
index 31d7a6ca12c7b..f11b7a6420a04 100644
--- a/ibis/backends/polars/compiler.py
+++ b/ibis/backends/polars/compiler.py
@@ -1170,7 +1170,16 @@ def execute_count_distinct_star(op, **kw):
 @translate.register(ops.ScalarUDF)
 def execute_scalar_udf(op, **kw):
     if op.__input_type__ == InputType.BUILTIN:
-        first, *rest = map(translate, op.args)
+        first, *rest = map(partial(translate, **kw), op.args)
         return getattr(first, op.__func_name__)(*rest)
     else:
         raise NotImplementedError("Only builtin scalar UDFs are supported for polars")
+
+
+@translate.register(ops.AggUDF)
+def execute_agg_udf(op, **kw):
+    args = (arg for name, arg in zip(op.argnames, op.args) if name != "where")
+    first, *rest = map(partial(translate, **kw), args)
+    if (where := op.where) is not None:
+        first = first.filter(translate(where, **kw))
+    return getattr(first, op.__func_name__)(*rest)
diff --git a/ibis/backends/polars/tests/test_udf.py b/ibis/backends/polars/tests/test_udf.py
index 8693c2a147977..9c1c0324f893c 100644
--- a/ibis/backends/polars/tests/test_udf.py
+++ b/ibis/backends/polars/tests/test_udf.py
@@ -54,7 +54,7 @@ def test_multiple_argument_udf(alltypes):
 @pytest.mark.parametrize(
     ("value", "expected"), [(8, 2), (27, 3), (7, 7 ** (1.0 / 3.0))]
 )
-def test_builtin(con, value, expected):
+def test_builtin_scalar_udf(con, value, expected):
     @udf.scalar.builtin
     def cbrt(a: float) -> float:
         ...
@@ -62,3 +62,18 @@ def cbrt(a: float) -> float:
     expr = cbrt(value)
     result = con.execute(expr)
     assert pytest.approx(result) == expected
+
+
+def test_builtin_agg_udf(con):
+    @udf.agg.builtin
+    def approx_n_unique(a, where: bool = True) -> int:
+        ...
+
+    ft = con.tables.functional_alltypes
+    expr = approx_n_unique(ft.string_col)
+    result = con.execute(expr)
+    assert result == 10
+
+    expr = approx_n_unique(ft.string_col, where=ft.string_col == "1")
+    result = con.execute(expr)
+    assert result == 1