Abstract Background Adaptive interpersonal emotion regulation (iER) is a vital tool for positive relations. During early adolescence, peer relations become increasingly important, making this age group a relevant target group for interventions promoting positive interactions with each other, yet no evidence-based intervention exists for iER and early adolescents specifically. Methods This randomised controlled trial (RCT) aims to test effectiveness and feasibility of a serious game training iER skills in early adolescents by comparing outcomes with a control group playing a game without psychoeducational content in a pre- and post-test design. German- and English-speaking early adolescents (10–14 years) are eligible for participation. IER skills improvement as assessed by a vignette task is the primary outcome and will be analysed with a chi-square test. Secondary outcomes include feasibility and acceptability, emotional competence, personal emotion regulation, gender, and sex. Discussion This RCT will test whether playing a serious game about iER strategies results in an improvement of iER skills and whether the game is feasible and acceptable for early adolescents with the ultimate aim to implement the game in schools and help early adolescents achieve positive peer relationships. Trial registration ClinicalTrials.govNCT04808102, Registered on 19 March 2021