<p>Illegitimate tasks are prevalent stressors in the workplace. However, no consensus has yet been reached regarding how illegitimate tasks impact creative behavior, even though illegitimate tasks have attracted much attention from researchers in recent years. To address this research gap, both studies explored the effect of illegitimate tasks on creative behavior of knowledge workers. Study 1 used a daily dairy study involving 104 knowledge workers, while Study 2 conducted a survey at two time points involving 574 knowledge workers. The results found that the relational energy knowledge workers experience during interactions with their family members moderated the indirect negative effect of illegitimate tasks on creative behavior through ego depletion, such that the indirect negative effect was weaker when the relational energy level was higher. These findings establish a new theoretical framework to enhance our understanding of the effect of illegitimate tasks on creative behavior through the lens of psychological energy, based on ego depletion theory and interaction ritual theory. Practically, these findings provide strategies for organizations to mitigate the negative effects of illegitimate tasks on knowledge workers’ creative behavior by alleviating ego depletion and fostering relational energy.</p>