Mantella baroni is a small frog, with adults measuring 22-30 mm in snout-vent length. The head, dorsum, and flanks are solid black. A yellowish rostral stripe is apparent, generally ending past the eye. The front limb and femur are yellow to greenish in appearance, with this coloration continuing up the flanks into a large, rounded flank blotch. These flank blotches will sometimes expand dorsally across the back and connect to the opposite side blotch, resulting in a more yellow dorsum (this is most often seen in M. baroni from the Andringitra region). Hindlimbs (tibia, tarsus, and foot) are orange with irregular black stripes. There are no flashmarks on the lower hindlimbs, in contrast to those of M. madagascariensis. The venter, throat, and limbs are black and marked with a few yellow to greenish, rarely blue blotches. The throat has one circular marking, but may be all black. The iris is black (Glaw and Vences 2007).
Mantella baroni was first described by Boulenger (1888). It is a member of the M. cowani species group (Goodman and Benstead 2003). There is a report of hybridization with M. cowani in the wild, at a locality near Antoetra (Andreone et al. 2005).