Sublethal dehydration can cause negative physiological effects, but recent studies investigating the sub-lethal effects of dehydration on innate immune performance in reptiles have found a positive correlation between innate immune response and plasma osmolality. To investigate whether this is an adaptive trait that evolved in response to dehydration in populations inhabiting water-scarce environments, we sampled free-ranging cottonmouth snakes (n=26 adult cottonmouths) from two populations inhabiting contrasting environments in terms of water availability: Snake Key (n=12), an island with no permanent sources of fresh water, and Paynes Prairie (n=14), a flooded freshwater prairie. In addition to field surveys, we manipulated the hydration state of 17 cottonmouths (Paynes Prairie n=9, Snake Key n=8) in a laboratory setting and measured the response of corticosterone and innate immune performance to dehydration with the aim of identifying any correlation or trade-offs between them. We measured corticosterone of cottonmouths at a baseline level and then again following a 60 min stress test when at three hydration states: hydrated, dehydrated and rehydrated. We found that innate immune performance improved with dehydration and then returned to baseline levels within 48 h of rehydration, which agrees with previous research in reptiles. Despite the frequent exposure of cottonmouths on Snake Key to dehydrating conditions, we did not find cottonmouths inhabiting the island to show a greater magnitude or more prolonged immune response compared with cottonmouths from Paynes Prairie. We also found a positive association between dehydration and corticosterone values.