The Udai or Udaj[1] (Ukrainian: Удай) is a river in Ukraine, a right tributary of the Sula, in the basin of Dnieper.

[2] The Udai finds its source near the village of Rozhnivka in Pryluky Raion, Chernihiv Oblast.

It is predominantly fed by melting snow (nival regime).

It freezes between November and early January, and it stays under the ice until March to mid April.

[2] The towns of Pryluky and Pyriatyn lie on the Udai,[2] as well as the urban-type settlements of Dihtiari, Varva, Ladan, and the village(selo) of Pisky-Udajsjki.