The beliefs children hold about their capabilities as readers are known to influence their reading achievement. The aim of this study was to extend previous work by examining trajectories of change in reading self-efficacy among primary school students (N = 1327) and the relations between the trajectories of self-efficacy and their hypothesized sources over 11 months. Using growth mixture modeling, we identified four trajectories of change in reading self-efficacy, involving increasing, stable, and declining trends. These trajectories of change in reading self-efficacy were associated with students’ varying experiences with the four sources of self-efficacy over time. Higher levels of mastery, verbal persuasion, and vicarious experiences and lower levels of physiological arousal were related to positive developmental trajectories of self-efficacy. Students with declining experiences of social sources of self-efficacy (i.e., verbal persuasions and vicarious experiences) had decreasing self-efficacy trajectories. These findings point to the importance of considering the variability in changes in reading self-efficacy and the interplay between changes in self-efficacy and sources of self-efficacy during primary school years, as well as the importance of monitoring these changes over time.