Drawing on conservation of resources theory, this study examined the moderating role of ethical leader behavior in the effects of daily perceived job insecurity on work outcomes the next day (i.e., work engagement and customer-directed helping) through occupational regret the next morning among frontline service employees working in adverse work situations (i.e., the coronavirus disease pandemic). Using experience sampling method, data were collected from 135 frontline service employees across five consecutive workdays. The results showed that daily perceived job insecurity had a negative indirect effect on work engagement and customer-directed helping the next day through (increased) occupational regret the next day in the morning. In addition, ethical leader behavior moderated the negative indirect effect of daily perceived job insecurity on next-day work engagement and customer-directed helping through next-morning occupational regret. Specifically, these negative effects were especially stronger among employees who had observed low levels of ethical leader behavior the previous day. The theoretical implications of the present findings for researchers and their practical implications for managers are discussed.