Michael Baius (1513 – 16 September 1589) was a Belgian theologian. He formulated the school of thought now known as Baianism.
He was born at Meslin L'Eveque near Ath in Hainaut as Michel De Bay, the son of Jean de Bay, a farmer. De Bay studied humanities in Brugelette and in Enghien and in 1533 he began studying philosophy at the Grand College het Varken of Leuven University. From 1535 he also studied theology at the Pope Adrian VI College. He was an excellent student and was ordained a priest in 1542, and was appointed director of the Standonck-College in Leuven.
In 1544 De Bay obtained his doctorate in philosophy and became a teacher at the College het Varken. In 1549 De Bay was appointed professor of scholastic philosophy. The following year, De Bay also obtained a license in theology and became president of the College Adrian, and also substituted for the professor of Holy Scripture, who was then absent at the Council of Trent. From February to August 1