Brian Leo John Hennessy, C.R., (7 January 1919 – 13 February 1997) was an American-born Roman Catholic bishop.
Hennessy, who was from Detroit, and raised in Hamilton, Ontario. He studied philosophy and theology at St. Peter's Seminary in London, Ontario and was ordained as a Catholic priest in North Bay, Ontario by Bishop Ralph Hubert Dignan on 30 July 1950. He served as headmaster at Scollard Hall in North Bay; principal of St. John's College in Brantford from 1965-1969; and as pastor of St. Joseph's Parish in Hamilton from 1969-1975.
He was ordained bishop on 14 May 1975 at the Cathedral of Christ the King in Hamilton by Bishop Paul Francis Reding. He was installed as the Bishop of the Roman Catholic Diocese of Hamilton in Bermuda 30 May 1975 and served the diocese to his retirement in 1995.