蔡共侯（？ - 前760年），即姬興，為春秋諸侯國蔡國君主之一，他為蔡釐侯兒子，承襲蔡釐侯擔任該國君主，在位期間為前761年—前760年，共2年。

Marquis Gòng of Cai (蔡共侯) (?–760 BC), born as Ji Xīng (姬興), was the ninth ruler of the State of Cai from 761 BC to 760 BC. He was the only known son of Marquis Yi of Cai (蔡夷侯), his predecessor. His reign only lasted for 2 years, which seem reasonable since his father's reign was 48 years making him an old man by the time he became Marquis. He was succeeded by his son.