@@ -34,9 +34,11 @@ use arrow::array::{
     StringArray, StringBuilder, StringViewArray,
 };
 use arrow::datatypes::{DataType, Field};
+use datafusion_common::arrow::datatypes::FieldRef;
 use datafusion_common::{Result, ScalarValue};
 use datafusion_expr::{
-    ColumnarValue, ScalarFunctionArgs, ScalarUDFImpl, Signature, Volatility,
+    ColumnarValue, ReturnFieldArgs, ScalarFunctionArgs, ScalarUDFImpl, Signature,
+    Volatility,
 };
 
 use crate::function::error_utils::{
@@ -75,6 +77,17 @@ impl ScalarUDFImpl for SparkConcatWs {
         Ok(DataType::Utf8)
     }
 
+    fn return_field_from_args(&self, args: ReturnFieldArgs<'_>) -> Result<FieldRef> {
+        // Spark skips null values after the separator, so only the separator controls nullability:
+        // https://github.com/apache/spark/blob/32f7299601108917fb01920a54e084595b7b3bf8/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala#L94-L96
+        let separator = &args.arg_fields[0];
+        Ok(Arc::new(Field::new(
+            self.name(),
+            DataType::Utf8,
+            separator.is_nullable(),
+        )))
+    }
+
     fn coerce_types(&self, arg_types: &[DataType]) -> Result<Vec<DataType>> {
         if arg_types.is_empty() {
             return Err(invalid_arg_count_exec_err("concat_ws", (1, i32::MAX), 0));
 
@@ -53,6 +53,11 @@ SELECT concat_ws(',');
 ----
 (empty)
 
+query TTT
+DESCRIBE SELECT concat_ws(',') AS result;
+----
+result Utf8 NO
+
 ## Null separator returns null
 query T
 SELECT concat_ws(NULL, 'a', 'b', 'c');
@@ -113,6 +118,12 @@ SELECT concat_ws(sep) AS result FROM VALUES (CAST(NULL AS STRING)), (',') AS t(s
 NULL
 (empty)
 
+query TTT
+DESCRIBE SELECT concat_ws(sep) AS result
+FROM VALUES (CAST(NULL AS STRING)), (',') AS t(sep);
+----
+result Utf8 YES
+
 ## ── Column expressions ──────────────────────────────────────
 
 ## concat_ws on columns
@@ -130,6 +141,12 @@ a,b
 c
 d
 
+query TTT
+DESCRIBE SELECT concat_ws(',', a, b) AS result
+FROM VALUES ('a', 'b'), ('c', CAST(NULL AS STRING)), (CAST(NULL AS STRING), 'd') AS t(a, b);
+----
+result Utf8 NO
+
 ## Scalar-only arguments over multiple rows (broadcast test)
 query T
 SELECT concat_ws(',', 'a', 'b') AS result FROM VALUES (1), (2), (3) AS t(x);