Children and young people in out-of-home care (OOHC) often display disproportionate levels of externalising behaviour problems compared to the general population, which are further linked with detrimental outcomes. Yet, despite similar levels of vulnerability, not all young people in OOHC develop these behaviours. The aim of this systematic review was to identify and synthesise knowledge on protective factors for externalising behaviour problems in young people between 0 and 19 years old residing in OOHC. A systematic search was conducted in PubMed, PsycINFO, Embase, CINAHL, and the Proquest Social Abstracts and Social Services databases, with 31 studies meeting inclusion criteria (n = 8310 individual participants across the studies). Results indicated that protective factors associated with fewer and/or lower levels of externalising behaviour problems included: individual (e.g., better self-concept), relationship (e.g., quality relationships with family members and peers), and contextual (e.g., higher neighbourhood income) factors. Given the short and long-term consequences associated with externalising behaviours, professionals working with young people in OOHC should focus on identifying protective factors that can be targeted in prevention and intervention efforts. This review indicated that strengthening individual, relational, and contextual factors may be most beneficial for reducing risk of externalising behaviours.