ABSTRACT: Atlantic cod is a key species of the North Atlantic ecosystem whose distribution will likely be affected by climate change. Although general temperature effects on reproduction are known, there is a dearth of information on population and individual level life history and reproductive plasticity responses to temperature change. We tested the hypothesis that the sperm of Atlantic cod of different genetic backgrounds (southern versus more northerly Newfoundland and Labrador) and of different environmental histories (reared in indoor tanks and fed a forage diet versus reared in sea cages and fed pellets) have different average plastic responses to temperature. Male reproductive performance was examined at 4 temperatures by measuring sperm swimming characteristics at 10 and 30 s after motility activation. Genetic origin had a larger effect on sperm swimming characteristics than past environmental history. Moreover, groups derived from the southern population exhibited a more pronounced positive mean reaction norm than the group derived from the Newfoundland and Labrador population. This represents an example of cogradient variation as genotypes accentuate the thermal phenotypic plasticity. Even though there were differences between the groups in sperm swimming characteristics, this is unlikely sufficient to affect successful reproduction at the temperatures tested unless under sperm competition. Thus, sperm performance should not be a limiting factor for reproduction under predicted increases in sea surface temperatures.