The Japanese weasel (Mustela itatsi), a small native carnivore in Japan, faces increasing threats to its survival from urbanization and competition from invasive species, yet its activity patterns remain poorly understood, particularly in urban environments. This study investigated Japanese weasel seasonal activity patterns and their temporal niche overlaps with sympatric carnivores in the Tama River Ecosystem Preservation Area in Tokyo. Using 12 camera traps across woodland, grassland, gravel and Sasa bamboo shrub habitat types, we monitored mammal activities from March 2021 to March 2022. Analysis of 121 independent detection events revealed the distinct seasonal activity pattern of Japanese weasels, namely diurnal activity during the warm season and cathemeral activity during the cold season. From temporal overlap analysis, temporal avoidance of larger carnivores influenced Japanese weasel activity patterns (overlap index Δ range: 0.219–0.560), serving as a strategy to minimize competitive interactions while maintaining access to resources. Temporal overlap between Japanese weasels and other sympatric carnivores was highest with red foxes (Vulpes vulpes) during the cold season and with raccoon dogs (Nyctereutes procyonoides viverrinus) during the warm season, and consistently lowest with raccoons (Procyon lotor) across both seasons. Reduced winter vegetation may increase Japanese weasel exposure to predation risk, potentially driving nocturnal activity shifts, although empirical validation is needed. Seasonal shifts in Japanese weasel activity may reflect established changes in its dietary preferences. We also deduce that multiple environmental drivers shape Japanese weasel activity patterns, including temperature, vegetation cover, and prey availability. Our findings suggest that temporal partitioning may serve as a crucial mechanism enabling Japanese weasels to coexist with other carnivores along urbanized river corridors, providing important insights for urban wildlife management and conservation strategies in increasingly urbanized landscapes.