Climate change has emerged as a critical driver of ecosystem dynamics, particularly in tropical biodiversity hotspots where vegetation productivity plays a pivotal role in ecological stability and carbon cycling. The study examines the long-term variability of Net Primary Productivity (NPP) in the Nilgiris Biosphere Reserve (NBR), India, over the period 1981–2019 using the Carnegie-Ames-Stanford Approach (CASA) model integrated with satellite-derived vegetation indices and climatic datasets. The analysis revealed pronounced spatial and seasonal heterogeneity in NPP, with higher productivity concentrated in the southwestern and northwestern regions, strongly influenced by monsoonal precipitation and orographic factors. Seasonal patterns indicated maximum NPP during the post-monsoon period and minimum during winter. Correlation analysis demonstrated that precipitation exerted the strongest positive influence on NPP, while air temperature and solar radiation exhibited negative relationships. Decadal assessments highlighted alternating phases of increasing and decreasing NPP, reflecting the dynamic response of vegetation to climatic variability. The findings underscore the sensitivity of NPP to changing climatic conditions and emphasize the necessity for ecosystem-based conservation and adaptive management strategies to safeguard the ecological integrity and carbon sequestration potential of the NBR.