The study addressed two main objectives: (i) to investigate disparities in motivation dimensions regarding extracurricular physical activity and (ii) to identify the influence of motivation on time spent in formal and informal physical activity. A sample of 704 adolescents (56% girls) from middle (46%) and high school (54%), with an average age of 14.88 ± 2.52, was assessed for different motivation dimensions using the Questionnaire of Motivation for Sports Activities (QMSA). Additionally, participants were categorized based on extracurricular physical activity practice. Multivariate analyses and multiple linear regressions were conducted to examine the effect of physical activity type on motivation dimensions and identify predictors of time spent in formal and informal physical activities, respectively. Results indicated that motivation varied significantly with extracurricular physical activity practice (p < 0.05), with students involved in extracurricular activities being more motivated. Sex and age differences were observed, with boys showing higher motivation in certain dimensions (achievement status (p < 0.001); group activity (p = 0.027); contextual (p = 0.004); technical improvement (p = 0.012) and older participants having lower scores in all dimensions. The influence of family and friends was a significant predictor only for boys in formal physical activity (p = 0.039). In terms of time spent in physical activity, group activity was a predictor for informal activities (p < 0.001), while technical improvement was a predictor for formal activities (p < 0.001), with notable sex differences. These findings underscore the importance of considering sex- and age-specific motivations when promoting physical activity among adolescents.