The psychological trait of self-control has been linked to interindividual differences in subjective well-being: Individuals with higher self-control report less negative affect, more positive affect, and higher life satisfaction. However, less is known about how much self-control fluctuates from day to day and how these fluctuations are related to subjective well-being. This intensive longitudinal study describes day-to-day fluctuations in self-control and investigates whether and how they are related to subjective well-being. A sample of 64 undergraduate students at the entry phase of university (M = 22.55 years, SD = 6.51, range = 18–53, 97% female) provided 1459 reports of their self-control and subjective well-being, collected every evening across three 9-day measurement bursts over 6 months. Participants’ self-control fluctuated substantially from day to day with less than 40% of the variability in daily self-control being attributable to interindividual differences in self-control. On days with higher self-control, participants reported less negative affect, more serenity, and higher life satisfaction. We found no relationship between self-control and vigor. The findings suggest that researchers need to go beyond current assessment practices and theories treating self-control as a stable trait to help develop tailored well-being interventions for everyday life.