The present study investigates the possible mechanisms involved in the link between daily job crafting and daily work engagement. Using self-determination theory, we hypothesize that daily job crafting is positively related to daily work engagement through momentary need satisfaction and momentary engagement. Additionally, using self-regulation theory, we predict that daily job crafting is negatively related to daily work engagement, through momentary energy depletion and (reduced) momentary work engagement. Participants from various occupational sectors (N = 66) responded to a daily diary questionnaire (N = 261) as well as momentary, task-related items (N = 1539) using a day reconstruction method at the end of each of four working days. The results of multilevel modeling were generally supportive of the hypotheses. We conclude that daily job crafting can have both positive and negative implications for daily work engagement, and discuss the practical implications of our findings.