ABSTRACT: The sei whale (Balaenoptera borealis) was once abundant in the Southwestern Atlantic Ocean. Following intensive whaling, the species was severely depleted and is now considered endangered due to its low numbers and prolonged history of exploitation. While some occurrence data exists for sei whales in the Southwestern Atlantic Ocean, the understanding on this species’ population ecology remains limited. In this study, we used information from visual surveys and from satellite telemetry to investigate sei whales' occurrence and movements in Santos Basin, off southeastern Brazil, in tropical waters of the Atlantic coast of South America. Between 2016 and 2024, 119 sei whale groups were sighted in the study area, with group sizes varying from 1 to 32 individuals (mean = 3.4 ± 4.1). Although the Brazilian coast is a presumed breeding ground, only three calves were recorded. A total of 26 sei whales were tagged during the study period, with track duration ranging from 3 to 53 days (mean = 22.5 ± 16.9). Total distance travelled ranged from 112.6 to 4,313.7 km (mean = 1,522.1 ± 1,179.5). A Hidden Markov Model (HMM) was fitted to telemetry data to predict behavioral states from location data. Models showed that sei whales predominantly engaged in Area Restricted Search (ARS) (53.6%) during the monitoring period, while transiting behavior was classified in 46.4% of the locations. Most of the locations estimated as ARS were in areas with marked topographic features such as submarine canyons along the continental slope of southeastern Brazil and close to seamounts. The occupancy time indicates frequent and yearly use of these areas, but movement patterns were not predictable and provide important information on use of breeding grounds. Since ARS occurred in areas considered productive, it may be related to opportunistic feeding, but more research is needed to describe this species behavior in tropical breeding grounds. This work brings an unprecedented and robust data set on this species, showing its regular occurrence and movements in the pelagic waters of the southeastern Brazilian coast during winter and spring.