The genus Synosis Townes, 1959 (Hymenoptera: Ichneumonidae: Metopiinae) currently comprises 18 described species. A redescription of the genus is presented and three new species are described and illustrated: S. diaguita new species, S. nigra new species, and S. zezei new species. Two previously known species, Synosis rubinus Alvarado and Rodriguez-Berrios, 2013 and S. townesi Alvarado and Rodriguez-Berrios, 2013 are recorded for the first time in Ecuador. The genus is recorded for the first time for Ecuador and Argentina. An identification key to the New World species is presented.