The potential long-term effects of altered salinity regimes on the bioenergetics of two ecologically important Kenyan mangrove crabs, Neosarmatium meinerti de Man, 1887 and Neosarmatium smithi H. Milne Edwards, 1853 were investigated in light of recent findings suggesting that groundwater redirection may alter salinity regimes in Kenyan mangroves. Although changes in groundwater may cause only small increases in salinities, these changes would be chronic and may impact crab populations already living above their optimal salinity. To assess potential impacts, fundamental physiological processes and hemolymph components were measured on animals acclimated to 16‰, 32‰, 48‰ and 65‰ for 4 weeks in a field laboratory. For comparative purposes, crabs were also sampled in the field. N. smithi survived poorly in all salinities except the control (32‰). Although high mortality in N. smithi did not allow for reliable estimations of an energy budget, mortality and osmoregulatory capacity shows that this species can osmoregulate for a limited time in elevated salinities (±1 week), but cannot withstand long-term hypersaline conditions. In contrast, N. meinerti survived well and was able to osmoregulate for 1 month in all salinity treatments. Nevertheless, their energy budget, was significantly reduced (to below 0) in the 65‰ treatment. Overall, this study shows that these two congeneric species exhibit different long-term responses to variations in salinity. However, they are both negatively effected by hypersaline conditions, suggesting that long-term alteration of mangrove salinity regimes may be detrimental for these ecologically important mangrove crab populations.