Predicting the response of populations to climate change requires knowledge of thermal performance. Genetic differentiation and phenotypic plasticity affect thermal performance, but the effects of sex and developmental temperatures often go uncharacterized. We used common garden experiments to test for effects of local adaptation (Florida versus Connecticut temperatures), developmental phenotypic plasticity (18oC vs. 22oC), and individual sex on thermal performance of the ubiquitous copepod, Acartia tonsa. Females had higher thermal tolerance than males in both populations, while the Florida population had higher thermal tolerance compared to the Connecticut population. An effect of developmental phenotypic plasticity on thermal tolerance was observed only in the Connecticut population. Ignoring sex-specific differences may result in a severe underestimation of population-level impacts of warming (i.e. - population decline due to sperm limitation). Further, despite having a higher thermal tolerance, Southern populations may be more vulnerable to warming as they lack the ability to respond to increases in temperature through phenotypic plasticity.