The Sitter is the largest tributary of the Thur river, and flows through the Swiss cantons of Appenzell Innerrhoden, Appenzell Ausserrhoden, St. Gallen and Thurgau. It starts on the north side of Alpstein massif in Appenzell Innerrhoden, and joins River Thur near the town of Bischofszell in Thurgau. It is 49 km long.
