Background: Obsessive-compulsive disorder (OCD) is a debilitating psychiatric disorder which affects up to 3% of children and adolescents. OCD in children and adolescents is generally treated with cognitive behavioural therapy (CBT), which, in more severely affected patients, can be combined with antidepressant medication. The TECTO trial aims to compare the benefits and harms of family-based CBT (FCBT) versus family-based psychoeducation/relaxation training (FPRT) in children and adolescents aged 8 to 17 years. This statistical analysis plan outlines the planned statistical analyses for the TECTO trial. Methods: The TECTO trial is an investigator-initiated, independently funded, single-centre, parallel-group, superiority randomised clinical trial. Both groups undergo 14 sessions of 75 min each during a period of 16 weeks with either FCBT or FPRT depending on the allocation. Participants are randomised stratified by age and baseline Children’s Yale–Brown Obsessive-Compulsive Scale (CY-BOCS) score. The primary outcome is the CY-BOCS score. Secondary outcomes are health-related quality of life assessed using KIDSCREEN-10 and adverse events assessed by the Negative Effects Questionnaire (NEQ). Primary and secondary outcomes are assessed at the end of the intervention. Continuous outcomes will be analysed using linear regression adjusted for the stratification variables and baseline value of the continuous outcome. Dichotomous outcomes will be analysed using logistic regression adjusted for the stratification variables. The statistical analyses will be carried out by two independent blinded statisticians. Discussion: This statistical analysis plan includes a detailed predefined description of how data will be analysed and presented in the main publication before unblinding of study data. Statistical analysis plans limit selective reporting bias. This statistical analysis plan will increase the validity of the final trial results. Trial registration: ClinicalTrials.gov NCT03595098. July 23, 2018.