The Treaty of Arras of 17 May 1579 was a peace treaty that was concluded between the Spanish Crown, represented by Alexander Farnese, Duke of Parma, and representatives of the County of Hainaut; the County of Artois; and the cities of Douai, Lille, Orchies and Arras, all of which were members of the Union of Arras, which had been formed on the 6 January 1579.
It was a separate peace that formally ended the state of war that had existed between those entities during the Eighty Years' War. The provinces and towns that had formed the Union of Utrecht (23 January 1579) continued the war.
After Don Juan of Austria, the royal governor-general of the Habsburg Netherlands, had broken with the States General of the Netherlands in July 1579 and resumed hostilities, the Union of Brussels initially maintained its common front against the government of King Philip II of Spain. However, after the death of Don Juan in October 1578, his successor, the Duke of Parma, approached a Catholic faction, known later as the "Malcontents", led by the stadtholder of Hainaut, Philip de Lalaing, 3rd Count of Lalaing, and his half-brother Emanuel Philibert de Lalaing (usually referred to as "Montigny") and convinced them to engineer a breach with the Prince of Orange, the leader of the States General, over the latter's policy of "religious peace". That led to the forming of the Union of Arras in January 1579. Its members (beside Hainaut the County of Artois and the cities of Lille, Douay and Orchies) then opened peace negotiations with Parma, which led to the signing of a separate peace.
The representatives of the parties to the Union of Arras had on 8 December 1578 (before the Declaration of 6 January 1579 was sworn to) agreed on a first draft of the treaty. That was followed by a second draft of 9 January 1579 and a third draft of 6 April 1579. Then, on 17 May 1579 the treaty was signed between the representatives of Parma and the members of the Union of Arras. But still the negotiations had not ended and Parma succeeded to wrest a number of further concessions from the treaty partners, which resulted in the version of 12 September 1579, which was ratified by King Philip and promulgated in Mons. It differs on appreciable and important points from the treaty as signed on 17 May.