The new species Dioon stevensonii, from the Rio Balsas basin spanning the states of Michoac´ an and Guerrero, Mexico, is described and illustrated. The de- scription of this species implies a recircumscription of the populations of Dioon that constitute the previously characterised D. tomasellii, which also includes populations located in Durango, Nayarit and Jalisco. Dioon stevensonii differs from its congeners in characters of both vegetative and reproductive structures - namely, leaflet con- tour shape, leaflet curvature and reflection of the megasporophyll tips. Despite its morphological affinities with D. tomasellii, complementary cladistic analyses of mo- lecular matrices indicate that D. stevensonii has closer phylogenetic affinities with the D. edule and D. spinulosum species groups, which are distributed along the Gulf of Mexico and Caribbean seaboards. The proposal of D. stevensonii rests on the re- evaluation of overlooked differences in morphological character states in herbarium material, corresponding to populations previously identified as D. tomasellii, which were confirmed after ex profeso field collections. The fact that such re-evaluation was prompted by a hypothesis of relationships based on molecular data for the entire genus Dioon, establishes the proposal of D. stevensonii as an unusual case of reciprocal illumination, where the morphological evidence provided confirmation of a molecular hypothesis, and not vice versa.