BackgroundSport participation has been associated with improved life prospects such as academic performance and employability prospects. As such, promoting sport participation might be a way to increase life prospects, especially for socially vulnerable youth because they are less physically active than their peers. However, the evidence for the causal effect of sport participation on these outcomes is still limited and little is known about factors that play a role in this possible effect. The aim of this study is four-fold. First, the causal effect of sport participation on life prospects is studied and the underlying mechanisms of this relation are explored. Secondly, the life experiences of the youngsters in the sport context, that may contribute to skill development, are studied. Thirdly, social conditions for a positive effect are explored, as sport is likely to have a positive effect under specific conditions. Fourthly, this study aims to provide insights on the elements of successful partnerships between youth care organisations and local sport clubs.Methods and designThis protocol reports on a mixed method study. An intervention that aims to increase the sport participation of socially vulnerable youth, between 12–23 years old, is implemented in three regions of a Rotterdam youth care organisation. The youngsters in the two control regions receive care-as-usual. The main outcome variables, collected via questionnaires, are the life prospect, sense of coherence and self-regulation skills of the youngsters after 6 and 18 months of follow-up. The Motivational Climate Scale is administered to explore the social conditions for a positive effect and interviews are conducted with sport coaches to explore their role in skill development. Interviews with the youngsters are conducted to gain insight on the life experiences that may lead to skill development. The elements of successful partnerships are collected during interviews with youth care professionals, sport coaches and other stakeholders in the sport context.DiscussionThe results of this study can support efforts of youth care organisations and local sport clubs to improve the life prospects of socially vulnerable youth through sport participation.Trial registrationTrialregister.nl NTR4621 Date of Registration: 2 June 2014