Future heat stress under six future global warming (ΔT GW ) scenarios (IPCC RCP8.5) in an Asian megacity (Osaka) is estimated using a regional climate model with an urban canopy and air-conditioning (AC). An urban heat ‘stress’ island is projected in all six scenarios (ΔT GW = +0.5 to +3.0 °C in 0.5 °C steps). Under ΔT GW = +3.0 °C conditions, people outdoors experience ‘extreme’ heat stress, which could result in dangerously high increases in human body core temperature. AC-induced feedback increases heat stress roughly linearly as ΔT GW increases, reaching 0.6 °C (or 12% of the heat stress increase). As this increase is similar to current possible heat island mitigation techniques, this feedback needs to be considered in urban climate projections, especially where AC use is large.