Self-concordant work motivation arises from one's authentic choices, personal values, and interests. In the present study, we investigated whether self-concordant motivation may fluctuate from one work-related task to the next. On the basis of self-determination theory, we hypothesized that momentary self-concordance buffers the negative impact of momentary work demands on momentary happiness. We developed a modified version of the day reconstruction method to investigate self-concordance, work demands, and happiness during specific work-related tasks on a within-person and within-day level. In total, 132 teachers completed a daily diary on three consecutive work days as well as a background questionnaire. The daily diary resulted in 792 reported work activities and activity-related work demands, self-concordance, and happiness scores. Multilevel analysis showed that—for most work activities—state self-concordant motivation buffered the negative association of work demands with happiness. These findings add to the literature on motivation and well-being by showing that the levels of self-concordance and happiness experienced by employees vary significantly on a within-day level and show a predictable pattern. We discuss theoretical and practical implications of the findings to increase employees' well-being.