A revision of the Mexican species of the genus Cohniella is presented. Each species is fully described and illustrated. Furthermore, species entries include specimen citation, discussion of diagnostic features and biogeography. A key to the Mexican species of Cohniella is presented. Two new species, Cohniella leptotifolia Cetzal & Carnevali and Cohniella yucatanensis Cetzal & Carnevali, are proposed. Cohniella brachyphylla (Lindl.) Cetzal & Carnevali is morphologically and distributionally recircumscribed. Finally, as part of our ongoing studies in this genus, a non-Mexican species related to C. ascendens, Cohniella teres, is synonymized under Oncidium helicanthum Kranzl. for which the combination Cohniella helicantha (Kranzl.) Cetzal & Carnevali is newly proposed. We provide a key to three closely related, easily confused species, C. aguirrei, C. ascendens, and C. helicantha.