Customer mistreatment is a prevalent negative event in the workplace. Beyond negative affect and rumination, the two most discussed passive responses toward customer mistreatment, we argue that counterfactual thinking is an agentic and functional cognitive approach employees may adopt to deal with customer mistreatment. Drawing on the functional theory of counterfactual thinking, we hypothesized that customer mistreatment would trigger both upward and downward counterfactual thinking, and the two counterfactual directions have countervailing effects on employees’ next-day job performance. In addition, individuals with higher self-esteem are more likely to adopt upward counterfactual thinking, and those with lower self-esteem are more likely to adopt downward counterfactual thinking when encountering customer mistreatment. One daily-diary study was conducted by sampling 205 call center representatives, and the results supported all our hypotheses. We discuss the implications and limitations of our study.