Given the high degree of autonomy and epistemic competencies required for university learning, students need the ability to make rational decisions rather than relying on heuristics and intuition. Previous cross-sectional findings indicate positive relations between rational thinking and university academic outcomes, but longitudinal research is lacking. This study aimed to investigate the predictive value of rational thinking for university academic outcomes and the temporal stability of rational thinking longitudinally. Participants were 124 students from a German university (Mage = 22.4 years; 84% women; 16% men). Data were collected in a longitudinal study with two measurement points over two semesters (Minterval = 5.9 months). At each point, rational thinking dispositions and skills, academic performance, and satisfaction with studying were assessed. Additionally, fluid intelligence was assessed at the first measurement point. Rational thinking predicted university academic outcomes in the following semester beyond the impact of fluid intelligence. Path analyses revealed that rational thinking skills significantly affected academic performance, whereas rational thinking dispositions significantly predicted student satisfaction but not vice versa. Our results showed no change in rational thinking dispositions or skills over two semesters, even when accounting for potential moderators such as academic progress, fluid intelligence, academic performance, and student satisfaction. In conclusion, our results highlight the impact of rational thinking on academic performance and student satisfaction. The findings underscore the need for targeted programs to cultivate rational thinking in students, as current university curricula seem to ineffectively promote this ability.