Climate change has significant effects on animals, being a major driver of the processes of speciation and extinction. Many environments will increase in the mean and variance of temperature.To adapt, ectotherms will have to increase their ability to thermally acclimate to novel temperatures. It is presently unclear how thermal acclimation evolves under climate change via natural selection. We are conducting one of the first experiments to test whether thermal acclimation can evolve via natural selection, and if so, whether selection is dependent on other traits such as upper thermal tolerance. Selection on thermal acclimation was measured across males and females from four populations of Tigriopus californicus, an intertidal copepod: two generations of each population were cultured under a common garden of 16.5 °C before F3 groups of full sibling larvae were split and developmentally exposed to low or high temperatures before thermal tolerance and reproductive output (a component of fitness) were measured in each sibship x temperature group. Selection on thermal acclimation was modeled as the linear relationship between fecundity and thermal acclimation. Southern populations showed more thermal tolerance and less thermal acclimation compared to northern populations while females exhibited greater acclimation and tolerance. The plasticity of thermal tolerance (acclimation) was under negative directional selection, and the strength of selection against acclimation was greater among more thermally tolerant sibships.