Pelagic longline fisheries in the southwest Atlantic are a major conservation concern for several threatened seabirds, including four species of great albatrosses: wandering albatross (Diomedea exulans), Tristan albatross (Diomedea dabbenena), southern royal albatross (Diomedea epomophora) and northern royal albatross (Diomedea sanfordi). The aim of this study was to examine the spatial and temporal variation in bycatch rates of these species, and to identify the contributing environmental and operational factors. We used data collected by observers on board pelagic longliners in the Uruguayan fleet in 2004–2011, and on Japanese vessels operating in Uruguay under an experimental fishing license in 2009–2011. Bycatch rates for northern and southern royal albatrosses were higher than expected based on previous reports, particularly over the shelf break. Wandering and Tristan albatrosses were caught predominantly in pelagic waters, where there are numerous fishing fleets from other flag states. Bycatch of great albatrosses was highest in April–November, with the peak for royal albatrosses in June–July, and for wandering and Tristan albatrosses in September–November. A range of vessel operational practices and habitat variables affected bycatch rates, among which setting time, moon phase, area and season are useful in terms of risk assessment, and in the development and improvement of conservation measures for these highly threatened species.