This study analyzes the decomposition of monthly rainfall series in Ondo State, Nigeria, over a 41-year period (1981–2021). Using time series decomposition techniques, the study separates the rainfall data into trend, seasonal, and irregular components to better understand its variability. The results reveal a clear seasonal pattern corresponding to the wet and dry seasons typical of the region. The trend component shows fluctuations without a consistent long-term increase or decrease, suggesting high interannual variability. The Holt–Winters exponential smoothing model was applied to assess the model’s fit and forecast rainfall for 60 months (2022–2026). Findings indicate that rainfall variability significantly influences agricultural productivity, water resources management, and flood risk mitigation in Ondo State. The study contributes to the growing body of knowledge on climatic variability and supports evidence-based planning for sustainable agricultural and environmental management.