We conduct experiments on an online platform to investigate the causal effect of gender discrimination on labor supply decisions. Controlling for the piece-rate wage, we find that workers who face explicit negative gender-discriminatory wage inequality supply substantially less labor compared with workers who face gender-neutral wage inequality. We also examine the effect of positive discrimination, differences between men and women, and the impact of implicit rather than explicit discrimination. We identify decreased work morale as the underlying mechanism. In addition, we provide survey evidence showing that discrimination in the field reduces work morale and labor supply, corroborating our experimental results.