助理主教是天主教、圣公会等实施主教制度的基督教宗派里，被委派去辅佐和拥有教区正权主教继承权的主教。

在天主教会内，助理主教是一种特别的辅理主教。但他们之间的分别是，助理主教拥有教区主教的自动继承权。这表示当教区主教因各种原因而退位时，助理主教便会自动成为教区的下一任主教。

助理主教的委任通常只会在教区主教将近退休年龄，或者教区主教健康状况出现问题下才会出现。当上述情况出现时，教宗便会委任助理主教，给予时间助理主教去熟悉教区事务，以便他日后管理教区。例如在1996年，时任天主教香港教区主教胡振中枢机将近75岁退休年龄时，便同时祝圣陈日君和汤汉为教区助理主教和辅理主教。在2002年，胡振中枢机病逝，陈日君便接任教区主教一职。

A coadjutor bishop (or bishop coadjutor) is a bishop in the Roman Catholic or Anglican churches who is designated to assist the diocesan bishop in the administration of the diocese, almost as co-bishop of the diocese. The coadjutor (literally, "co-assister" in Latin) is a bishop himself, and while also appointed as vicar general, is given authority beyond that ordinarily given to the vicar general, making him co-head of the diocese in all but ceremonial precedence. In modern times, the coadjutor automatically succeeds the current bishop of a diocese upon the latter's retirement, removal or death.