The well-being of young people in relation to their school performance has received increased attention in recent years. However, there is a lack of knowledge about the longitudinal and reciprocal relationship between adolescents’ subjective well-being and their academic achievements. The current study examined the bidirectional relationship between subjective well-being and academic achievement across two timepoints (T1 and T2) during the course of mid to late adolescence, i.e.,in school year 9 (age 15), and school years 11–12 (ages 17–18). The study also investigated variation in the association as afunction of adolescent gender. Data on subjective well-being and teacher-assigned school grades of 723 adolescents (48.7% girls) residing in Sweden were analyzed by estimating a series of cross-lagged path models. The findings suggest gender differences in the relationship as no associations were found among boys. Support for a bidirectional relationship between the constructs was only found for girls. For girls, higher subjective well-being at T1 was associated with higher academic achievements at T2, while higher academic achievements at T1 was associated with lower subjective well-being at T2. These findings highlight that the subjective well-being of adolescent girls may be important for their ability to perform at school, but their academic achievements may also inflict negatively on their subjective well-being.