Purpose: This study evaluates whether physical activity (PA) and sedentary behavior (SB) are influenced by a common mediating relationship. Method: A total of 1,552 participants in 3rd to 12th grade completed an online survey that included assessments of PA at school (PAS), PA at home (PAH), and SB as well as a battery of psychosocial variables (i.e., attraction to PA and perceived competence). Results: Perceived competence had a direct positive effect on PA and a negative effect on SB. These associations were consistently (but partially) mediated by attraction to PA; however, the indirect effect (IE) of perceived competence was stronger for PAH (IE = 0.27, p < 0.05) than for PAS (IE = 0.07, p < 0.05), or SB (IE = 0.13, p < 0.05). Conclusions: This study revealed some direct effects and IEs of perceived competence on PAS, PAH, and SB through attraction to PA as the mediator. PA and SB may be influenced by some common underlying psychosocial mediators.