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Marine environments have increased in temperature by an average of 1°C since pre-industrial (1850) times [ 1 ]. Given that species ranges are closely allied to physiological thermal tolerances in marine organisms [ 2 ], it may therefore be expected that ocean warming would lead to abundance increases at poleward side of ranges and abundance declines toward the equator [ 3 ]. Here, we report a global analysis of abundance trends of 304 widely distributed marine species over the last century, across a range of taxonomic groups from phytoplankton to fish and marine mammals. Specifically, using a literature database, we investigate the extent that the direction and strength of long-term species abundance changes depend on the sampled location within the latitudinal range of species. Our results show that abundance increases have been most prominent where sampling has taken place at the poleward side of species ranges, and abundance declines have been most prominent where sampling has taken place at the equatorward side of species ranges. These data provide evidence of omnipresent large-scale changes in abundance of marine species consistent with warming over the last century and suggest that adaptation has not provided a buffer against the negative effects of warmer conditions at the equatorward extent of species ranges. On the basis of these results, we suggest that projected sea temperature increases of up to 1.5°C over pre-industrial levels by 2050 [ 4 ] will continue to drive latitudinal abundance shifts in marine species, including those of importance for coastal livelihoods. • Global-scale analysis of marine species shows abundance changes linked to warming • Increases at poleward sides of species ranges reflect new ecological opportunities • Declines at equatorward sides show failure to adapt to rapid climate change • Results imply future warming will impact further on abundance of marine species Globally, the oceans have warmed by over 1°C during the last century. Hastings, Rutterford, et al. show how marine species are responding. At the poleward sides of species ranges, populations are increasing as warming makes habitat more favorable, but at equatorward sides, populations are shrinking as seas become too warm. [ABSTRACT FROM AUTHOR]