Higher temperatures linked to climate change lead to people feeling increasingly heat stressed compromising their health and reducing economic activity. In this paper we assess the potential economic impact of heat stress on working people in urban Malaysia by analysing the loss in productivity that they associate with heat stress. We found that nearly every respondent (99%) from a sample of 514 drawn from an online survey sometimes feels heat stressed and also less productive as a result. The median number of days in a year on which people felt their productivity had been compromised because of heat stress was 29. On those days half of the respondents felt their work capacity had been at least halved. The estimated median annual loss from reduced productivity was 257 € nearly 10% of respondents’ median annual income. Respondents who work in mentally challenging jobs are more affected by heat than those in physically intense jobs. They also receive the highest incomes, so suffer the highest losses. Our research suggests that the real economic costs of heat has probably been under-estimated because most research has so far focused on people working in physically intense outdoor jobs or those performed in very hot environments.