Foraminiferal assemblages were studied in 93 samples collected in the lagoon of Araruama and in the adjacent area. The lagoon of Araruama is one of the largest hypersaline lagoons in the world, with a salinity range of 52–65 ‰. Historical reports show that the lagoon has been hypersaline since at least the 16th century. Foraminiferal assemblages are dominated by miliolids, mainly Triloculina oblonga, and rotaliids, with Ammonia tepida and the less abundant Cribroelphidium excavatum var. selseyense. Textulariids are almost absent. This assemblage is similar to those usually reported from hypersaline lagoons with sandy carbonate-rich sediments and from salt marshes. A high proportion of aberrant tests was observed. Anthropogenic stresses do not seem to be responsible for these morphological abnormalities, which are attributed to high salinity conditions and to changes of salinity. However, the higher proportion of Ammonia tepida in the more impacted northern part of the lagoon is probably due to human impact.