Here we describe five new species of Synergus Hartig with partially smooth mesopleurae that were obtained from oak cynipid galls on different species of Quercus L. in Mexico: Synergus cibriani sp. nov., Synergus gilletti sp. nov., Synergus longimalaris sp. nov., Synergus longiscapus sp. nov. and Synergus striatifrons sp. nov. To date, this trait has been observed in at least 18 American species, 16 recorded in North America (United States of America) and only 2 in Central America (Panama): Synergus elegans Nieves-Aldrey and Medianero and Synergus laticephalus Nieves-Aldrey and Medianero. The type material and additional material of Synergus punctatus Gillette from the United States have been examined and compared with S. gilletti sp. nov. The type material and additional material of S. elegans from Panama and Synergus citriformis (Ashmead) from the United States have also been examined and compared. This study records Synergus elegans in Mexico for the first time and classifies it as a syn. nov. of S. citriformis. Descriptions and diagnoses for the new species and a key to their identification and differentiation from their closest related species are given. Their distribution and host associations are also commented on.