The Union of Arras (Dutch: Unie van Atrecht, Spanish: Unión de Arrás) was an alliance of a number of provinces in the southern part of the Habsburg Netherlands, who (dissatisfied with the religious policies of the Prince of Orange and the States General of the Netherlands) signed a declaration on 6 January 1579 about their intent to offer a vigorous defense of the Roman Catholic religion against what they saw as the encroachments of the Calvinists in other provinces.