Aim To understand the latitudinal patterns in body size distributions of north-eastern Pacific bivalves, as well as the factors influencing those patterns.Locations The north-eastern Pacific continental shelf from 5 °S latitude to 70 °N latitude.Methods We used data on the body size and latitudinal ranges of 915 species of north-eastern Pacific marine bivalves to test for the presence of interspecific latitudinal size trends. We tested for trends in both mean body size as well as variance in body size using randomization tests.Results Overall, there is no significant linear relationship between either mean size or variance in size and latitude. Rather, there is a mid-latitude trough and peak in mean body size and variance in body size, respectively, with changes in the sign of the correlations occurring at provincial boundaries. A significant decreasing trend in mean size with latitude exists within the tropical Panamic province. In contrast, variance in body size shows a significant increasing trend with latitude within the tropics, and a significant decreasing trend with latitude in the cold temperate and Arctic provinces. For north-eastern Pacific bivalves, latitudinal trends in mean body size and variance in body size show very little correlation with latitudinal trends in species richness. Close to an order of magnitude difference in species numbers between tropical and polar latitudes does not appear to significantly affect either mean size or variance in size. Changes in mean body size associated with biogeographic boundaries are significantly larger than changes elsewhere along the range.Main conclusions The spatial distribution of major environmental barriers along the north-eastern Pacific margin plays a major role in structuring the latitudinal distribution of body size in marine bivalves along this coast.