Using self-determination theory as a framework, we aimed to study the relationships between perceived need support and need satisfaction with self-determined motivation and extracurricular physical activity intentions in the physical education (PE) classroom, including sex and out-of-school sport participation as moderators. Additionally, we aimed to test whether a need-supportive classroom environment in PE moderates these associations. Participants were 1259 students (556 males) aged between 12 to 16 years (Mage = 13.46 years; SD = 0.74) from 77 PE classes. At the student level we found (a) need satisfaction to predict positively autonomous motivation and negatively amotivation, and (b) autonomous motivation to predict positively and amotivation to predict negatively intentions to undertake extracurricular physical activities. At the classroom level, in need-supportive classes males benefit more than females in terms of increased autonomous motivation while females benefit more than males in terms of decreased amotivation. Finally, class-level perceived need support moderated (i.e., attenuated) the negative association between need satisfaction and amotivation and between amotivation and intentions. These results suggest a buffering role that a need-supportive classroom environment may have on students’ motivation and behavior.