Many teachers report high levels of occupational stress. Teachers' basic need satisfaction is essential for teachers' well-being at work. Social support from school principals is assumed to play an important role for teachers' basic need satisfaction. However, the underlying mechanisms of the relationship between social support from school principals and teachers' basic need satisfaction are mostly unknown. Previous research suggests that job demands and job resources may play an important mediating role. Therefore, we examine whether teachers' perceived job demands and job resources serve as mediators between social support from the school principal and teachers' basic need satisfaction. Using longitudinal data of N = 1071 teachers over the course of one school year, we applied structural equation modelling to test the hypothesised mediation model. Results showed that the relationship between social support from the school principal and teachers' basic need satisfaction was mediated by teachers' perceived job demands and job resources. Particularly, the job demand 'unclear organisational conditions' and job resource 'social support from colleagues' indicated the strongest effects on teachers' basic need satisfaction. These findings emphasise the responsibility of school principals to provide social support to their teachers and create a well-structured and supportive workplace. In doing so, school principals contribute to a work environment in which teachers can thrive.