The Lunain is a 51.4 km (31.9 mi) long river in the Yonne and Seine-et-Marne departments in north-central France. It is a right tributary of the Loing. Its source is in Yonne, less than 3 km (1.9 mi) south-west of Égriselles-le-Bocage. It joins the Loing at Épisy in Seine-et-Marne.