Temperature plays a multidimensional role in host-pathogen interactions. As an important element of climate change, elevated world temperature resulting from global warming presents new challenges to sustainable disease management. Knowledge of pathogen adaptation to global warming is needed to predict future disease epidemiology and formulate mitigating strategies. In this study, 21 Phytophthora infestans isolates originating from seven thermal environments were acclimated for 200 days under stepwise increase or decrease of experimental temperatures and evolutionary responses of the isolates to the thermal changes were evaluated. We found temperature acclimation significantly increased the fitness and genetic adaptation of P. infestans isolates at both low and high temperatures. Low temperature acclimation enforced the counter-gradient adaptation of the pathogen to its past selection and enhanced the positive association between the pathogen’s intrinsic growth rate and aggressiveness. At high temperatures we found that pathogen growth collapsed near the maximum temperature for growth, suggesting a thermal niche boundary may exist in the evolutionary adaptation of P. infestans. These results indicate that pathogens can quickly adapt to temperature shifts in global warming. If this is associated with environmental conditions favouring pathogen spread it will threaten future food security and human health and require the establishment of mitigating actions.