The Union of Atrecht (French: Arras) was an accord signed on January 6, 1579 in Atrecht (Arras), under which the southern states of the Spanish Netherlands, mostly today in Wallonia and the Nord region in France, expressed their loyalty to the Spanish king Philip II and recognised the landlord, Don Juan.

The parts that signed it, were:

• Hainaut
• Artois
• Lille, Douai and Orchies (Walloon Flanders )

These parts finally ended up in support of the Spanish king Philip II together with Limburg (not to be confused with the provinces in present-day Belgium and the Netherlands) and Luxembourg, of which Alexander Farnese, the duke of Parma, started to his conquest of the separating parts that joined the Union of Utrecht.