Drifting reefs of Sargassum (a brown alga) are used by a variety of pelagic seabirds in the western Atlantic Ocean. We examined gut contents from 964 individuals of 39 seabird species collected 5 to 60 km off the coast of North Carolina for evidence of Sargassum use. Sargassum pieces or Sargassum-associated prey were found in nine of 10 Procellariiformes species and less frequently among Charadriiformes (12 of 25 species). No Sargassum-associated prey was found in Pelecaniformes examined, but observational data indicated that Atlantic tropicbirds (Phaethon lepturus and P. aethereus) and Masked Boobies (Sula dactylatra) commonly foraged over Sargassum. Four species were considered Sargassum specialists, having frequencies of occurrence >25% and high volumes of Sargassum-associated prey: Audubon's Shearwater (Puffinus lherminieri), Royal Tern (Thalasseus maximus), Bridled Tern (Onychoprion anaethetus), and Red-necked Phalarope (Phalaropus lobatus). Seven species fed in Sargassum to a lesser extent, and nine species had ingested Sargassum pieces, but contained no Sargassum-associated prey. It is likely that other seabird species forage regularly over Sargassum, as our conclusions are based on relatively small sample sizes taken during random sampling in the open ocean. Our conservative analysis and extensive observational data indicate the Sargassum community is critical for feeding for some western North Atlantic seabirds. Degradation of Sargassum habitats by oil development, harvest, and/or ocean acidification would undoubtedly have negative effects on fitness of these birds.