Reproductive barriers limit gene flow and drive population divergence. For internal fertilizers, sperm morphology plays an important role in reproductive barriers, as successful fertilization depends on how well sperm perform in the female environment. Specifically, sperm length must be adapted to fit the female reproductive tract and storage organs. In species where sperm competition occurs, i.e. where multiple males compete to fertilize a female’s eggs, selection pressure can favor an optimal sperm length, reducing variation over time. Additionally, variation in sperm length may result in different optima in locally adapted populations or ecotypes that may facilitate further divergence. We investigated sperm length in species and ecotypes of Littorina, a genus of promiscuous marine snails with internal fertilization. Various Littorina species mate inter-specifically, where sperm length differences might prevent hybridization across species borders. Additionally, several Littorina species have ecotypes adapted to different shoreline environments where reproductive barriers like sperm length divergence might play a role in reinforcing these barriers. Due to their promiscuity, sperm competition probably plays a role in Littorina, and sperm length variation can give insights in sperm competition intensities. Littorina snails have two types of sperm, eusperm which is the fertilizing sperm, and parasperm. This study examined eusperm length in four species (L. fabalis, L. littorea, L. obtusata, L. saxatilis), and two different ecotypes in both L. fabalis and L. saxatilis. The ecotypes of both L. fabalis and L. saxatilis differed in eusperm lengths, suggesting that this trait may be involved in prezygotic reproductive barriers between ecotypes of these species. Among-species differences in eusperm length variation were observed and may be a result of different sperm competition intensities.