Thermal acclimatization and phenotypic plasticity strongly influence the behaviour and epidemic potential of Phytophthora palmivora, a major pathogen of coconut. To dissect short-term thermal responsiveness and acclimation capacity, two isolates originating from contrasting coconut ecosystems- a low-altitude warm-plain isolate (CBP) and a high-altitude hill-region isolate (TN) were subjected to stepwise warming (25–35 °C) and cooling (25–15 °C) regimes across four successive generations per temperature step. Fourth-generation cultures were evaluated for colony growth, aggressiveness, sporulation, and pathogenicity. Temperature exerted a significant effect on all traits (p < 0.001), with strong interaction between isolate origin, acclimation status, and temperature. Contrary to expectations based on ecological origin, the hill-region isolate TN displayed a broader thermal performance curve, superior colony expansion, higher aggressiveness, and sustained sporulation at both thermal extremes, whereas CBP exhibited reduced performance under stress. Multi-generational acclimation enhanced thermal tolerance in both isolates-most notably in TN, allowing maintenance of virulence even at 15 °C and 35 °C. Acclimated cultures produced larger lesions, faster tissue maceration, and more severe disease on seedlings compared with unacclimated controls. The acclimation effect was physiological and reversible, as re-isolated acclimated cultures lost their enhanced performance when returned to optimal temperatures. These results demonstrate that P. palmivora possesses substantial thermal plasticity, enabling rapid adjustment to fluctuating environments without requiring genetic change. Isolates from thermally variable hill ecosystems may have a competitive advantage under future warming scenarios. Such acclimatization capacity has important implications for coconut disease dynamics under climate change, underscoring the need for climate-responsive surveillance and management strategies.