© 2020, Springer Nature B.V. Thailand’s low-lying capital city Bangkok is a dense metropolis, one of Asia’s rapidly growing ‘megacities’, and home to over 10 million residents. Bangkok’s location on the sinking Chao Phraya Delta means it has a high susceptibility to river flooding. However, the possible contribution of marine incursion to river flooding is not as widely recognised and needs further consideration. We therefore re-examine the historically documented floods in AD 1785 and 1983. Available evidence suggests that the superposition of different flood behaviours was important during both events. Attention is drawn to the evidence for, and contributory roles played by, astronomical spring tides, meteorological tides, storm waves and possibly storm surge. These marine effects can exacerbate river floods caused by seasonal peak flows in the tidal lower Chao Phraya River during the early north-east monsoon. For future management of flood risk, it is therefore necessary to recognise that Bangkok has a compound flood-risk profile that includes both hydrometeorological and marine influences. The significance of marine influences on Bangkok’s flood risk is likely to increase in future with continuing eustatic sea-level rise and subsidence of the Chao Phraya Delta.