The Treaty of Mersen or Meerssen, concluded on 8 August 870, was a treaty to partition the realm of Lothair II, known as Lotharingia, by his uncles Louis the German of East Francia and Charles the Bald of West Francia, the two surviving sons of Emperor Louis I the Pious. The treaty followed an earlier treaty of Prüm which had split Middle Francia between Lothair I's sons after his death in 855.

Lothair II was the king of Lotharingia from 855 until his death in 869. He was the second son of Emperor Lothair I and Ermengarde of Tours. He was married to Teutberga, daughter of Boso the Elder.