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.

The treaty is referred to in some Western European historiographies as the third major partition of Francia, all of which took place from August 843 to August 870, through the treaties of Verdun, Prüm and Mersen's. It was followed by the Treaty of Ribemont.