A new diminutive species of the diadramous goby genus Sicydium is described fram 346 specimens collected between 10 and 800 m elevation in Costa Rica. The new form is almost always syntopic with its cogener, S. altum. The new species is compared to other Costa Rican species (S. altum, S. salvini and S. cocoensis) with special emphasis on details of the premaxillary dentition, which is shown to be of primary importance in understanding the relationships of Sicydium species. Specimens of Sicydium sp. (Honduras), S. hildebrandi (Colombia) and S. plumieri (Puerto Rico) were al so examined. On the basis of the study material, two distinct lineages of Sícydium which differ sharply in tooth structure and mode of replacement appear to exist.