OBJECTIVE To test if motivational regulations (i.e., amotivation, external, introjected, identified, and intrinsic) mediate the association between depression symptoms and moderate-to-vigorous physical activity (MVPA) in young adults. METHOD A total of 319 young adults from Montreal, Quebec participating in the Nicotine Dependence in Teens (NDIT) study provided self-report data on depression symptoms at age 20 (SD=0.7 years) and motivational regulations at age 24 (SD=0.7 years). MVPA was assessed using a self-report questionnaire at age 20 (i.e., at the same time motivation was measured) and using accelerometers one month later. Multiple mediation analysis with bootstrapping was employed to analyze the data. RESULTS Controlling for age, sex and self-reported MVPA at age 20 years; introjected regulation was a significant mediator of the association between depression symptoms and MVPA (R(2) adj=0.12; point estimate=-0.0011; BCa CI=-0.15 to -0.01). CONCLUSION Depression symptoms may undermine controlling forms of motivation, resulting in decreased MVPA. Intervention strategies may be needed to address depression symptoms in young adults to promote sustained MVPA.