Abstract A review of the current knowledge on the diversity of inquiline oak gall wasps (Hymenoptera: Cynipidae: Cynipinae) from Colombia is conducted for the first time. A new species, Synergus dawnus Lobato-Vila and Pujade-Villar, is formally described and illustrated, and an undescribed specimen from the same genus is diagnosed and discussed. New biological, morphological, and distribution data of the only two previously known species from this country ( Synergus colombianus Nieves-Aldrey, 2005 and Synergus pedroi Pujade-Villar, Lobato-Vila, and Fernández-Garzón, 2017), as well as an identification key to the Colombian species of Synergus Hartig, 1840, are provided. The morphological variability of the Colombian species of Synergus , especially of S. pedroi , is discussed.