Exasticolus xmatkuilensis sp. n. (Hymenoptera: Braconidae: Homolobinae) is described from the State of Yucatan, Mexico. It is the first species of the genus known from the Yucatan Peninsula. A key to the species of the genus Exasticolus van Achterberg is added. Exasticolus rosenbergi (Viereck, 1911) is taken from the synonymy with E. fuscicornis (Cameron, 1887) and treated as a valid species.