Tsu (tsō) [key], city (1990 pop. 157,177), capital of Mie prefecture, S Honshu, Japan, on Ise Bay. It is a commercial and manufacturing center, with glass, machine, and food-processing factories. In the city are a feudal castle, testimony to its bygone role as a castle town, and several large Buddhist temples. Tsu also has Mie National Univ. and Mie Prefectural Univ.