Background This longitudinal study investigated the development of moral competence among adolescents and examined the influence of selected environmental factors, including religion, education, parents, Physical Education teacher, peers, and media. Examining these determinants is particularly relevant in the context of contemporary social transformations associated with increasing moral ambiguity, shifting value orientations, and a growing prevalence of risk behaviours among young people. These processes may affect adolescents’ psychosocial development, well-being, and health-related behaviours, highlighting the importance of moral competence as a factor relevant to public health.Methods A longitudinal study using the diagnostic survey method was conducted among students from randomly selected secondary schools in the Wielkopolska (Greater Poland) region of Poland. The study sample comprised 209 adolescents aged 15–19 years (57.2% boys and 48.8% girls) who were followed throughout four years of secondary education between 2020 and 2024. Data were collected in three consecutive measurement waves over a 40-month period. Moral competence was assessed using the validated Lind’s Moral Competence Test and the author-developed Moral Influence Scale. Given the non-normal distribution of the data, nonparametric statistical procedures were applied, including McNemar’s test to assess changes over time, Spearman’s rank-order correlations to examine associations between variables, and regression analysis to identify predictors of moral competence.Results The findings indicated a significant increase in the proportion of adolescents representing a medium level of moral competence between the second and third measurement waves (p = 0.014). Correlation analyses showed that at lower levels of moral competence, moral reasoning was weakly associated with selected external influences, particularly religion and the PE teacher; however, these associations weakened as competence levels increased. Regression analysis identified religion as the only significant predictor of medium-level moral competence.Conclusions The findings support stage-based models of moral development, suggesting that adolescents’ moral reasoning becomes increasingly autonomous over time. The results emphasize the importance of moral education and value-based reflection. Adolescents also identified the physical education teacher as an influential factor, highlighting the potential role of physical education contexts in fostering moral development.