Significance We report how much water flooded Bangladesh during the 2020 wet monsoon period and explain why the 2020 monsoonal flooding started unusually early, lasted longer, and was more severe than flooding in other years. The laser-ranging instrument on GRACE Follow-On spacecraft detected the size and timing of this unusual flooding event, and we found that the 2020 flooding was worsened by soils saturated earlier than usual by a premonsoon cyclone. We highlight the importance of including the antecedent soil moisture condition in the present flood forecasting and warning practices in Bangladesh.