Adiantum alan-smithii (Pteridaceae) is described from Chiapas, Mexico. It is similar to A. raddianum, and previously identified as such, however it can be distinguished by its yellow-farinose indusia, the proximal acroscopic pinnule of each pinna usually overlapping the main rachis, flabellate segments, and rachises that are weakly flexuose distally. In contrast, A. raddianum has non-farinose indusia, the proximal pinnule of each pinna does not overlap the main rachis, and segments are obovate to rhomboid. The new species is described, illustrated, and a key to species of the A. raddianum group in Mexico is provided. This contribution adds an additional endemic species of Adiantum to the flora of Mexico, and is part of our ongoing effort to improve the circumscription of A. raddianum, one of the most over-applied names in the Neotropical fernflora.