Abstract The fern genus Polystichum Roth (Dryopteridaceae) remains one of the most difficult genera taxonomically. In this work, I provide an identification guide to the Costa Rican Polystichum species in the context of an evaluation of characters and evolution in the genus in the Neotropics. Key characters are those of petiole and pinna-rachis scales, ultimate segment shape, and indusium. There are 12 species in the country, of which three are allopolyploids (P. lilianiae Barrington, P. orbiculatum (Desv.) J. Remy & Fee, and P. talamancanum Barrington). The species' distributions vary: while five are widespread in Tropical America, two are at the southern end of their range (P. alfaroi (Christ) Barrington and P. speciosissimum (A. Braun ex Kunze) R. M. Tryon & A. F. Tryon), and two are at the northern end (P. dubium (H. Karst.) Diels and P. nudicaule Rosenst.). Three are confined to Costa Rica and Panama: P. concinnum Lellinger ex Barrington and the allopolyploids P. lilianiae and P. talamancanum. Essent...