Achi-Su (), also known as Achisu (), is an urban locality (an urban-type settlement) in Karabudakhkentsky District of the Republic of Dagestan, Russia. As of the 2010 Census, its population was 1,679.
Within the framework of administrative divisions, the urban-type settlement of Achi-Su is incorporated within Karabudakhkentsky District as Achi-Su Settlement (an administrative division of the district). As a municipal division, Achi-Su Settlement is incorporated within Karabudakhkentsky Municipal District as Achi-Su Urban Settlement.