Objectives: This manuscript describes the rationale and protocol for an on-line, school-based randomised controlled trial of "Projecte d'Educació Postural a l'Escola" (Postural Education in School Project) (PEPE) which aims to prevent non-specific low back pain and promote healthy postural habits in children. The study will examine the effects of the intervention on back care knowledge, daily postural habits (DPH) and the correct use of backpacks. Design: Randomised controlled trial informed by self-determination theory (SDT) and the social-ecological model (SEM). Setting: A total of 880 students attending 10 primary schools (five experimental and five control) from different Majorcan cities will participate in this study. Method: The intervention (on-line 12-week intervention for class teachers, physical education teachers and families) will be conducted by the research staff; a control group will continue their routine activities. Physical fitness (PF) levels will be measured by a modified version of the ALPHA Fitness test battery for children while back care knowledge and other study variables will be measured by a self-reported on-line questionnaire at baseline and post-intervention. The primary outcomes will be PF, backpack weight, back care knowledge and DPH. Conclusion: Findings will provide a comprehensive understanding of the short-term effects of this school-based intervention on back care knowledge, DPH, the correct use of backpacks and PF levels in Majorcan children.