Seabirds are good indicators of wider biodiversity and where they assemble in large numbers signifies sites important to many marine faunal species. Few such large assemblage sites have been identified and none in pelagic waters has been identified in the tropical Atlantic Ocean despite their importance for resident seabirds and those ‘on passage’ during migration. Here, we identify the likely location of just such an assembly site and provide preliminary information about the distribution of pelagic seabirds around Ascension Island in the tropical South Atlantic Ocean using a combination of trans-equatorial seabird migrant tracking data, records of at-sea surveys and land counts of seabirds returning from foraging trips. We found that waters north–north-west of Ascension Island are used more often by seabirds than those south and east of the island. Three-fifths of the species recorded in the assembly site breed at mid- or high-latitudes and some of these migratory seabirds stopover possibly to wait for favourable winds that facilitate onward flight. Our findings are important because to the best of our knowledge no seabird assembly sites have previously been identified in tropical Atlantic Ocean pelagic waters. We provide evidence to support the aspirations of the Marine Reserves Coalition that waters in the vicinity of Ascension Island should be recognised as a sanctuary for marine wildlife and we highlight an area that is worthy of further targeted investigation.