Coral reefs are threatened by anthropogenic climate change, corroborating the urge to find thermally tolerant species, the reasons behind their thermal tolerance and refugia for threatened populations. The strong dependence of the coral host on the symbiotic energy supply makes the role of the symbionts for thermal tolerance especially interesting and hence the focus of this study. Transplantations to high variance environments have been suggested as possible sources for an increased thermal tolerance. Ko Racha in the Andaman Sea on the west coast of Thailand, represents such an area with west sides exposed to large amplitude internal waves and the South-West monsoon. For the first time a nine-month reciprocal cross-transplant experiment followed by an ex-situ heat stress experiment (control: 29°C, heat: 32°C, 25 days thermal stress) was conducted at Ko Racha with the coral Porites lutea. The goal was to test for symbiont acclimation to the new habitat and if that acclimation maintains under heat stress. The symbiont’s health and fitness, metabolic rates and fitness traits were measured for signs of acclimation and role in thermal tolerance. Transplants showed an acclimation in symbiont densities and pigmentation after the transplantation period. Acclimation was mostly maintained throughout the heat stress with symbiont densities, pigmentation, mitotic index and protein content showing an environmental imprint in the East to West- and in photosynthetic efficiency in West to East corals. The response to thermal stress appears to be a combination of an adaption of the fragments to their respective native environment combined with an acclimation to the transplantation destination. This study shows that symbionts of cross-transplanted P. lutea in the Andaman Sea can acclimate to novel environments and obtain local higher thermal tolerance in selected parameters, and therefore give hope for successful transplantations with the goal of increased thermal tolerance in the future.