Previous studies have mainly focused on the detrimental effects of illegitimate tasks as ubiquitous workplace stressors while ignoring the appraisal measures for such tasks. The term "illegitimate" is used by employees to describe the alignment of a task with their job role rather than the inherent qualities of the task itself. Thus, drawing on the transactional theory of stress, this study examines the moderating effect of the stress mindset on the relationship between illegitimate tasks and the appraisal of such tasks. On this basis, this study further explores when cognitive appraisal mediates the effects of illegitimate tasks on coping behaviors (task crafting and cyberloafing). Data were collected from 285 employees from an energy company in Shandong, China, by using a time-lagged research design. The findings indicate that employees react differently to illegitimate tasks depending on their stress mindset. Specifically, for employees with a stress-is-enhancing mindset, illegitimate tasks induce their challenge appraisal, which leads to task crafting. In addition, illegitimate tasks induce hindrance appraisal in employees with a stress-is-debilitating mindset, which leads to cyberloafing. In practice, this research study suggests that when illegitimate tasks cannot be eliminated, organizations should consider employees' stress mindset when assigning such tasks.