Stomachs from 511 Raja velezi and 340 Mustelus henlei captured as by-catch in the commercial trawling fishery (2010–2012) were analysed to examine diet composition, ontogenetic shifts and degree of dietary overlap between species life stages in the Pacific Ocean of Costa Rica. Shrimps were the most important prey categories in the diet of R. velezi, while teleosts and cephalopods dominated the diet of M. henlei. Diet comparisons between different stages of R. velezi and M. henlei revealed clear ontogenetic dietary shifts: crustaceans (mainly shrimps, crabs and stomatopods) dominated the diet of immature individuals, and adults had a higher proportion of teleosts. The results suggest that R. velezi is an epibenthic predator that specializes in shrimps during early life stages, and to a lesser extent, teleosts as it matures, while M. henlei is an opportunistic predator with a highly diverse diet consisting of teleosts, cephalopods, shrimps and stomatopods. This study also found little evidence of dietary overlap between species or life stages and suggests that intra- and interspecific competition between R. velezi and M. henlei may be reduced by: (1) diet specialization in immature stages of R. velezi, (2) ontogenetic dietary shifts between immature and mature individuals, (3) prey-size selectivity in larger individuals of R. velezi and (4) differences in depth utilization in overlapping geographical regions.