| Füsinger Au Fysing Å | |
| Source1 Coordinates: | 54.5925°N 9.5311°W |
| Mouth Coordinates: | 54.5306°N 9.6261°W |
| Subdivision Type1: | Country |
| Subdivision Type2: | State |
| Length: | 35km (22miles) |
The Füsinger Au (pronounced as /de/; also Loiter Au, pronounced as /de/; Danish: Fysing Å / Løjt Å) is a river in the north of Schleswig-Holstein (Danish: Slesvig and Holsten respectively), Germany.
The Füsinger Au starts north of Idstedt (Danish: Isted), flows through the lakes Idstedter See and Langsee (Danish: Isted Sø and Langesø respectively), passes the village Loit (Danish: Løjt) and discharges into the Schlei (Danish: Slien) near Füsing (a district of Schaalby) (Danish: Fysing and Skålby).