Based on ego depletion theory and interaction ritual theory, we tested the effect of illegitimate tasks on creative behavior of knowledge workers using two studies. Study 1 aimed to test the effect of illegitimate tasks on creative behavior using a 10-day daily dairy study with a sample of 104 knowledge workers. Study 2 aimed to test the effect of illegitimate tasks on creative behavior using a questionnaire survey conducted at two points in time with a sample of 567 knowledge workers. The results of both studies were highly convergent. The relational energy knowledge employees experience during interactions with their family members moderated the indirect negative effect of illegitimate tasks on creative behavior through ego depletion, such that this indirect negative effect was stronger when the relational energy level was low and weaker when the relational energy level was high. The results deepen the understanding of the mechanisms underlying the effect of illegitimate tasks on creative behavior from the perspective of psychological energy, and offer practical insights into managing knowledge workers’ creative behavior.