The present study evaluated the seasonal restructuring of freshwater fish assemblages in the Cherupuzha tributary of the Chaliyar River, Kerala, under monsoonal influence. Fish sampling conducted during pre-monsoon, monsoon, and post-monsoon seasons in 2025 recorded 26 species belonging to 8 orders and 15 families. Cypriniformes and Cyprinidae were the dominant taxonomic groups throughout the study period. Seasonal diversity analysis revealed higher diversity and evenness during the pre-monsoon season, while monsoon assemblages exhibited increased dominance and reduced diversity due to hydrological disturbance and habitat instability. Relative abundance patterns demonstrated clear seasonal turnover in fish community structure. Multivariate analyses, including NMDS ordination and cluster analysis, revealed distinct seasonal segregation of assemblages, with monsoon communities showing comparatively lower similarity to pre- and post-monsoon assemblages. The findings highlight the significant role of monsoonal hydrology in shaping fish assemblage dynamics and community organization in tropical freshwater ecosystems of the Western Ghats. The study provides baseline ecological information useful for biodiversity conservation and management of tributary ecosystems within the Chaliyar River basin.