diff --git a/pygeoapi/util.py b/pygeoapi/util.py
index b60e187a08..2accb96c49 100644
--- a/pygeoapi/util.py
+++ b/pygeoapi/util.py
@@ -423,7 +423,7 @@ def json_serial(obj: Any) -> str:
             return obj.decode('utf-8')
         except UnicodeDecodeError:
             LOGGER.debug('Returning as base64 encoded JSON object')
-            return base64.b64encode(obj)
+            return base64.b64encode(obj).decode('ascii')
     elif isinstance(obj, Decimal):
         return float(obj)
     elif type(obj).__name__ in ['int32', 'int64']:
diff --git a/tests/other/test_util.py b/tests/other/test_util.py
index 7cb321019d..e857e6dad3 100644
--- a/tests/other/test_util.py
+++ b/tests/other/test_util.py
@@ -166,6 +166,20 @@ def test_json_serial():
     d = uuid.UUID('12345678-1234-5678-1234-567812345678')
     assert util.json_serial(d) == '12345678-1234-5678-1234-567812345678'
 
+    # Valid UTF-8 bytes round-trip as a plain string
+    assert util.json_serial(b'hello') == 'hello'
+
+    # Non-UTF-8 bytes must produce a base64 *str* (not bytes), so that the
+    # result can be serialised to JSON without a TypeError.
+    raw = b'\x80\x81\x82'
+    result = util.json_serial(raw)
+    assert isinstance(result, str), "json_serial must return str, not bytes"
+    import base64 as _b64
+    assert result == _b64.b64encode(raw).decode('ascii')
+    import json as _json
+    # Must be embeddable in a JSON document without raising
+    _json.dumps({'data': result})
+
     with pytest.raises(TypeError):
         util.json_serial('foo')