Background: Teacher wellness affects teaching performance and students' academic achievement; hence, teacher wellness matters and should be a concern to educational researchers and practitioners. There are several teacher wellness approaches, but no one of them specifically focuses on a Sports-based Physical Activity programme. Objective: To evaluate the effects of a Sports-based Physical Activity programme on teachers' relatedness, stress and motivation towards physical activity. Method: Middle school teachers (n = 32) volunteered to participate in the research as programme participants (n = 12) or non-programme participants (n = 20). The Sports-based Physical Activity programme included 7 weeks of training in Catchball, a team sport and a tournament as the culminating event. The study adopted a mixed-methods design: a quasi-experimental design collecting quantitative data from online surveys and a case study design collecting qualitative data from interviews with the programme participants, non-participants and the school principal. Results: Repeated-measures analysis of variance revealed that the Sports-based Physical Activity programme was effective in relatedness, F(1, 30) = 5.16, p = 0.031, [eta][subscript p][superscript 2] = 0.147, and in one of the six motivation subscales, external regulation, F(1, 30) = 4.23, p = 0.048, [eta][subscript p][superscript 2] = 0.124. The qualitative analysis findings supported the programme's contribution to teachers' relationship, stress and exercise motivation. Conclusion: Overall, the programme was effective in promoting teachers' relationships in school, controlling levels of teachers' stress and fostering teachers' motivation towards physical activity. Further studies are required to examine the impact of a Sports-based Physical Activity approach in diverse school contexts and its impact on student learning.