Detailed morphological and rbcL sequence analyses are presented for two species of Gelidium collected along the Pacific coast of Costa Rica. Although relatively well known in the tropical eastern Pacific, Gelidium sclerophyllum W. R. Taylor had previously been collected only once in Costa Rica. Four current G. sclerophyllum collections were found to have identical rbcL sequences and differed from a partial sequence for an isotype specimen at only one base pair. Phylogenetic analyses revealed that G. sclerophyllum and G. floridanum W. R. Taylor are geminate sister species. Development of the female reproductive system and cystocarps were observed to follow the usual patterns described for Gelidium species. Spermatangial development in G. sclerophyllum was also observed for the first time and revealed the cutting off of spermatia by transverse divisions as is characteristic of Gelidiales. The new species, Gelidium microglossum Grusz & Freshwater, was also collected and described. This is a small species with a prostrate habit, generally compressed axes of variable width, and clavate to obphyriform ultimate branches. Distinctive characteristics include spatulate tetrasporangial branchlets, medullary cells often forming a meshlike network, variably oriented rhizines, and an unusual attachment haptera structure. Phylogenetic analyses resolve G. microglossum within Gelidium, but placement within the genus remains uncertain.