Bevacizumab, a humanized monoclonal antibody that targets vascular endothelial growth factor and inhibits angiogenesis, can cause hypertension. Although blood pressure generally rises in winter and falls in summer, seasonal patterns of bevacizumab-related hypertension (BRHT) remain unclear. We investigated these patterns using Japanese Adverse Drug Event Report data from July 2007 to June 2025. Cases where bevacizumab was reported as a “suspected drug” were identified using the Medical Dictionary for Regulatory Activities preferred terms “hypertension,” “blood pressure increased,” or “systolic blood pressure increased.” Monthly relative BRHT frequencies were calculated by dividing BRHT reports by the total adverse event reports and compared with mean monthly temperatures obtained from the Japan Meteorological Agency. Seasons were defined as spring (March–May), summer (June–August), autumn (September–November), and winter (December–February), and were visualized using mosaic plots. Between April 2004 and September 2025, 985,999 reports were registered, of which 956 met the BRHT criteria. The median time-to-onset (TTO) was 21.0 days (interquartile range, 7.0–45.0). The Weibull scale parameter (α) was 40.8 (95% confidence interval [CI]: 37.4–44.5), and the shape parameter (β) was 1.06 (95% CI: 0.99–1.13). BRHT reports peaked in September and October and were least frequent in summer. No significant differences were observed in either TTO or clinical outcomes across seasons of adverse event onset. BRHT occurred more frequently in autumn, particularly in September and October, than in winter. Healthcare professionals should be aware of this seasonal risk and ensure appropriate blood pressure monitoring and preventive measures during autumn.