Significance Phytoplankton play essential roles in marine food webs and global biogeochemical cycles, yet the responses of individual species and entire phytoplankton communities to anthropogenic climate change in the coming century remain uncertain. Here we map the biogeographies of commonly observed North Atlantic phytoplankton and compare their historical (1951–2000) and projected future ranges (2051–2100). We find that individual species and entire communities move in space, or shift, and that communities internally reassemble, or shuffle. Over the coming century, most but not all studied species shift northeastward in the basin, moving at a rate faster than previously estimated. These pronounced ecological changes are driven by dynamic changes in ocean circulation and surface conditions, rather than just warming temperatures alone.