Introduction. Decentering describes the ability to voluntarily adopt an objective self-perspective from which to notice internal, typically distressing, stressors (e.g. difficult thoughts, memories, and feelings). The reinforcement of this skill may be an active ingredient through which different psychological interventions accrue reductions in anxiety and/or depression. However, it is unclear if decentering can be selectively trained at a young age and if this might reduce psychological distress. The aim of the current trial is to address this research gap. Methods and analysis. Adolescents, recruited from partnering schools in the UK and the EU (n = 48 per group, age range = 16-19 years), will be randomised to complete of five-weeks of decentering training, or form an active control group that will take part in in light physical exercise and cognitive training. The co-primary training outcomes include a self-reported decentering inventory (i.e. the Experiences Questionnaire) and the momentary use of decentering in response to psychological stressors, using experience sampling. The secondary mental health outcomes will include self-reported inventories of depression and anxiety symptoms, as well as psychological wellbeing. The initial statistical analysis will use mixed-model analysis of variance (ANOVA) to estimate the effect of training condition on self-rated inventories across three timepoints: baseline, mid-intervention and post-intervention. Additionally, experience sampling data will be initially interrogated using hierarchical linear models. Ethics and dissemination. This study was approved by the Cambridge Psychology Research Ethics Committee, University of Cambridge (PRE.2019.109). Findings will be disseminated through typical academic routes including poster/paper presentations at (intern)-national conferences, academic institutes and through publication in peer-reviewed journals.