Background: Different studies reveal an association between parent involvement, metacognition, and academic achievement; however, the majority analyse the developmental process of acquisition in experimental or quasi-experimental designs with students enrolled in child education. Adolescence is an important stage in personal and academic development. Given the complexity of learning, and according to the metacognitive and affective model of self-regulated learning (Efklides, 2011, "Educational Psychologist," 46, 6), different types of metacognitive processes are relevant for the optimal development of individuals in academic contexts. At the same time, individuals need stimuli from the environment, based on observation of their own and other's behaviour, as well as through communication and interaction with others. Aim: The present study examined the relations among parent involvement, metacognitive strategies, and academic achievement to investigate the mediational role of metacognition in the relation between parent involvement and academic achievement. Sample: The participants were 1,398 high school students from Spain (47% female, M = 12. 5 years). Method: Multiple multilevel mediation analyses with Monte Carlo confidence intervals were used for measuring within-subjects effects at the student level and between-subjects effects at the class level. Parent involvement (perception of support, organization, and interest in the educational process; expectations; school relationship; time of support with homework) and metacognition were measured by questionnaires, whereas academic achievement was assessed using the end-of-term grades obtained by students for nine subjects. Results: First, there was a significant direct effect of most parent involvement constructs on the mediational and dependent variables at both the within and between levels. Expectations had the highest predictive power on academic achievement. On the other hand, metacognitive strategies were an important mediator for all parent involvement constructs at both levels of analysis. Conclusions: These results highlight the importance of metacognition during early adolescence and suggest that parent involvement is crucial for the future development of educational models.