BackgroundAdolescents with intellectual disabilities have an increased risk of developing academic, social, and psychological problems compared with non-disabled peers. These difficulties might have an impact on the implicit theories—or so called mindset—of these youth. Youth with a fixed mindset believe that their attributes are static while youth with a growth mindset believe their attributes are malleable. A growth mindset can positively affect the academic and psychosocial development of youth and can be stimulated by so called ‘mindset interventions’. Nevertheless, mindset interventions specifically adapted to adolescents with intellectual disabilities are non-existing.Methods/designThe aim of the present study is to conduct a randomized controlled trial (RCT) to test the effectiveness of the online mindset intervention “The Growth Factory” aimed to teach adolescents with intellectual disabilities a growth mindset and thereby positively impacting their psychosocial development. The RCT targets adolescents (12–23 years) with mild to borderline intellectual disabilities (IQ 50–85) admitted to residential care or special education. Participants will be individually randomized to the intervention (n = 60) or control (n = 60) group. The intervention group will individually participate in the six sessions of “The Growth Factory” and the control group will receive care as usual. Primary outcome will be mindset. Empowerment, behavior problems, self-esteem, treatment motivation, therapeutic alliance, challenge seeking, and the impact of social exclusion will be included as secondary outcome measures. Moreover, moderation (i.e., intervention satisfaction, IQ, age, baseline mindset, gender) and mediation effects will be investigated. Self-reported and mentor assessments will be administrated at baseline, post-test and at three (except mentor assessment) and six months follow-up.DiscussionThis paper describes the design of a RCT examining the effectiveness of the online mindset intervention “The Growth Factory” aimed to empower adolescents with intellectual disabilities. If effective, “The Growth Factory” makes an important contribution to the treatment and psychosocial development of adolescents with intellectual disabilities in residential care and special education. Due to the online approach, implementation will be efficient and cost-effective and therefore the intervention “The Growth Factory” can be used on large scale.Trial registrationDutch Trial Register NTR5460. Registered 2 October 2015.