Antônio de Castro Mayer

| offstyle = Your Excellency | relstyle = Bishop }}

Antônio de Castro Mayer (20 June 1904 – 25 April 1991) was a Brazilian prelate of the Roman Catholic Church. A Traditionalist Catholic and ally of Archbishop Marcel Lefebvre, he was Bishop of Campos from 1949 until his resignation in 1981.

He is often described as incurring the automatic canonical penalty of excommunication for participating in the 1988 illicit consecration of four bishops of the Society of St. Pius X (SSPX), as did Lefebvre and the four bishops then ordained. He was mentioned in the letter from Cardinal Gantin on July 1 but not in the letter from Pope John Paul II on July 2. Provided by Wikipedia