To examine the bi-directional relationships between negative affectivity (i.e., depression, anxiety, and anger) and adolescents’ physical activity in nearly real time. Twenty-six adolescents (Mage = 15.67 ± 1.56 years; 57.7% male) were asked to complete 80 self-report measures of their negative affect (depression, anxiety, and anger) via a smartphone app and wear an accelerometer as an objective assessment of their physical activity over 20 consecutive days. Negative fixed effects emerged for within-person depression and moderate-to-vigorous physical activity (MVPA) as well as for within-person anger and MVPA. Further, there were significant random effects for each of the within-person negative affect variables and MVPA. Study findings highlight the importance of considering individual differences in the association between negative affect constructs and physical activity.