A strong decline in species richness from the equator to the poles is a common feature of Earth’s biodiversity. However, little is known about how phenotypic diversity varies across the same latitudinal gradient. Here, we examine body shape diversity in marine fishes across latitudes and explore the role of time and evolutionary rate in explaining the diversity gradient. Marine fishes' occupation of upper latitude environments has increased substantially over the last 55 million years. Latitude strongly affects the rate of body shape evolution and its disparity. Fishes in the highest latitudes exhibit nine times the rate of body shape evolution and one and a half times the disparity compared to equatorial latitudes. The more dynamic evolution of body shape may be due to increased ecological opportunity in polar and subpolar oceans due to (1) the evolution of anti-freeze proteins in certain temperate clades that allowed them to invade regions of cold water, and (2) periodic environmental disturbances driven by cyclical warming and cooling in upper latitudes. Our results suggest that decreasing water temperature, through its effects on the activity levels of fishes, may have elevated the relative frequency of body shapes associated with less-active lifestyles.