A new species of pontoniine shrimp, Periclimenes siankaanensis, is described from the Mexican Caribbean coast. The new species shares morphological features with P. patae Heard & Spotte,1991, P. antipathophilus Spotte, Heard & Bubucis, 1994, and P. mclellandi Heard & Spotte, 1997, all species belonging to the “iridescens” complex, and with P. platalea Holthuis, 1951, of the tropical east Atlantic. Periclimenes siankaanensis differs from P. patae and P. mclellandi by its large size, pereopods 3–5 with 2 or 3 spines on propodal inner margin and the presence of bifid dactyl, and is distinguished from P. antipathophilus by its smaller size, entire endite on the second maxilla, pereopods 2 of same shape, and by the lack of tooth on the movable finger. In addition, P. siankaanensis can be separated immediately from P. platalea by having the second pereopods equal in shape and length, and bifid dactyl present on pereopods 3–5. Periclimenes siankaanensis inhabits shallow water Thalassia testudinum seagrass meadows and sandy bottoms, while the other species here compared are associated with gorgonians in deeper zones.