<p>Solar radiation is a key energy input governing regional hydroclimatic processes, yet its role in shaping climate variability in Bangladesh remains inadequately quantified. This study aims to evaluate the influence of solar radiation on hydroclimatic variability in Bangladesh using a Vector Autoregressive (VAR) modelling and trend analysis. A seasonal VAR framework was employed to analyse temporal variations between solar radiation and the Standardized Precipitation–Evapotranspiration Index (SPEI), which reflects anomalies in climatic water balance (precipitation minus atmospheric evaporative demand). Findings reveal that the wettest months are primarily June through September, with occasional rainfall extending into October and November; the driest months occur from December to February. Modified Mann–Kendall tests and Sen’s slope estimator show significant upward trends in both solar radiation and SPEI, suggesting a gradual shift toward warmer and more sub-humid moisture regimes in most regions across Bangladesh. Impulse Response Function (IRF) analysis reveals that solar radiation shocks have an intense and persistent impact on SPEI and rainfall, with responses extending beyond two decades. Variance decomposition indicates that solar radiation accounts for approximately 13% of the observed variability in surface warming. The results suggest that solar radiation has both short-term and long-term effects on Bangladesh’s hydroclimatic variability. Overall, these results underscore the pivotal role of solar radiation in shaping hydroclimatic variability, providing valuable insights for climate policy, disaster risk reduction, and sustainable environmental management in Bangladesh.</p>