Heat reduces labor productivity and output in formal manufacturing but little is known about its impacts on the earnings and welfare of workers in the informal sector that comprise 82% of the labor force in low-income and lower-middle-income countries. This study reports the results from daily surveys of nearly 400 workers in two slums in Delhi for a month in the summer of 2019. Every degree Celsius increase in wet bulb temperature was associated with a fall in gross earnings of 13(±3.5) percentage points, a fall in earnings net of work-related expenditure of 19(±4.5) percentage points, an increase in the self-reported probability of sickness of the worker or a family member of 6(±0.5) percentage points, and a decrease in the probability that a worker went to work of 2(±0.5) percentage points. Net earnings were 40% lower during the two heatwaves that occurred during the study period. Over 320 million informal-sector workers in low-income and lower-middle-income countries are currently exposed to temperatures similar to those observed in this study.