Saint Acarius (died 27 November 639) was bishop of Doornik and Noyon, which today are located on either side of the Franco-Belgian border.
Acarius was a pupil of Saint Columbanus around 594. While the date of his election is not known, it is assured that he was bishop in 627. He was a fervent furtherer of the apostle Saint Amand, but despite his influence could not avoid the banishment of the latter by King Dagobert I.
Apparently, Acarius had great influence on the spreading of Christianity in Belgic Gaul. Soon after his death in 639, he was venerated as a Saint.