Hucho taimen, the world's largest salmonid, is a potadromous species that is listed as endangered in Mongolia. While mature individuals are known to have extended longitudinal movements of over 90 km along main river channels, details of the seasonal movements and residency of individuals within and between tributary habitats have been largely undocumented. Therefore, the current research aimed to detect and quantify the seasonal distances moved by adult taimen (65–96 cm; n = 10) within and between the Onon River (Amur River basin) and a major tributary (Balj) over 12 months using passive acoustic telemetry. The median distance moved by taimen in spring was 17.4 km (n = 6), in summer 9.1 km (n = 4), autumn 4.7 km (n = 9) and winter 0.4 km (n = 8). However, there were no statically significant differences amongst these median seasonal home range sizes. Two taimen traversed between the Onon River and the Balj tributary during the study period, recording overall home ranges of 44.5 km and 126.1 km. One of these individuals moved twice, exiting the tributary in autumn and returning again in spring where it remained for 36 days, while the second taimen moved only once into the tributary in spring and re-entered the main channel in late summer after 85 days. Another two taimen entered surrounding smaller tributaries and recorded home ranges of 26.1 km and 29.2 km, while all remaining individuals were detected moving only within the waterway where they were originally captured and released (0.8–20.1 km). Taimen movements within tributaries such as the Balj can be extensive (<60 km), as they provide access to important spawning, feeding and overwintering habitats as well as refuge from adverse thermal and hydrological conditions. Thus to enhance population recovery, it is essential that current and future management and conservation efforts include preserving or restoring the ecological integrity and hydrological connectivity of these critical tributaries and the main river channel throughout the Onon and Amur River basins and across the remaining distribution of this endangered species.