The present study was designed to examine the mediating effect of self-efficacy on relations of middle school students' four achievement goals with their perceptions of two motivational climates and in-class physical activity in physical education. The four achievement goals (mastery-approach, mastery-avoidance, performance-approach, and performance-avoidance), perceptions of the motivational climate (mastery- and performance-involving climates), and self-efficacy were measured in a sample of 194 students (93 boys, 101 girls) in a public school. Students' in-class physical activity was assessed using Actical Accelerometers. A series of multiple-regression analyses supported the mediating effect of self-efficacy on the relationships among students' mastery-approach goal, perceived mastery-involving climate, and physical activity.