The ability to retain educationally relevant content in a readily accessible state in memory is critical for students at all stages in schooling. We hypothesized that a high degree of stress in mathematics courses can threaten students’ mathematics self-concept and lead to a motivation to forget course content. We tested the aforementioned hypothesis by recruiting students from a college course on multivariate calculus. Students were asked to report their ongoing stress in the course. The forgetting rate was assessed by comparing students’ final exam performance against their performance for a subset of the same final exam items 2 weeks later. We found that among students with a strong mathematics self-concept, a higher amount of ongoing weekly stress during the course was associated with increased forgetting of course content and a higher report of avoidant thinking about the course. Neither of these associations was found among students with a weaker mathematics self-concept. Our results provide evidence for a scientific account of the affective and motivational forces that shape why students forget educationally relevant content. We discuss the various educational practices that cue forgetting and make recommendations for reducing motivated forgetting in the classroom.