Ecological niche theory predicts coexisting species will segregate spatially, temporally or trophically to limit competition. Few studies have investigated niche segregation in multiple dimensions during the breeding season in sympatric, morphologically similar seabird species. Moreover, these studies showed discrepancies between theoretical predictions and observations. We tested the hypothesis of niche segregation during breeding between 2 sympatric small-sized seabirds, the blue petrel Halobaena caerulea and the thin-billed prion Pachyptila belcheri, by quantifying foraging niche and trophic differences. Combining at-sea GPS and isotopic (stable carbon and nitrogen) data, we assessed spatial, foraging habitat and trophic segregation. We found strong latitudinal and longitudinal segregation between species during incubation. Mean maximum distance from the colony during foraging trips was ~2330 and ~1300 km for blue petrels and thin-billed prions, respectively. Foraging habitat segregation varied during incubation, with blue petrels foraging close to the ice edge and prions foraging in oceanic areas far from ice. Although sample size was low, there was also some evidence for spatial and habitat segregation during chick-rearing, with prions foraging in areas with negative sea surface height anomalies. Trophic segregation was revealed by differences in stable isotopes, with blue petrels feeding on higher trophic level prey than prions (mean plasma δ15 N : 9.6 ± 0.4 and 9.0 ± 0.3‰, respectively). Spatial distribution and diet are the primary segregation mechanisms, and patterns of segregation may result from competitive exclusion rather than niche specialization. Spatial and trophic segregation may have evolved to minimize competition, allowing co-existence of these 2 abundant sympatric breeding species.