The prevalence of eating disorders among adolescents has increased in recent years, particularly following the COVID-19 pandemic. Family-based treatment (FBT) is widely considered a first-line intervention for restrictive eating disorders which are persistent disturbances in eating behavior or related behaviors that result in altered consumption or absorption of food and significantly impair physical health or psychosocial functioning with a key feature of restriction. However, its effectiveness varies, and the predictors of treatment success remain unclear. This systematic review aimed to examine the effectiveness of family-based interventions for adolescents with restrictive eating disorders and to identify key predictors of treatment outcomes, with a particular focus on early weight gain. Family-based interventions were defined as an umbrella term referring to structured treatments involving active caregiver participation in management of eating disorder symptoms, including manualized FBT and adapted family-involved therapeutic approaches. A systematic search was conducted across selected databases (CINAHL, Medline, PubMed, and PsycINFO), yielding 265 including 22 studies in the final review. Study quality was assessed using CASP. Data were extracted on study design, participant characteristics, intervention types, outcome measures, and reported predictors of treatment response. Outcomes of interest included weight restoration, early weight gain, eating disorder severity, and family functioning. Due to heterogeneity in study designs, interventions, and outcome measures, a narrative synthesis was conducted. Family-based interventions, particularly FBT and its adaptations, were associated with improvements in weight restoration, eating behaviors, and family functioning. Early weight gain emerged as the most consistent predictor of positive treatment outcomes, although its definition varied across studies. Parental involvement and communication, were also associated with improved outcomes. Overall, findings suggest that family-based interventions are associated with improved outcomes for adolescents with restrictive eating disorders, although effectiveness is influenced by multiple interacting factors. Early weight gain appears to be an important predictor of treatment response. However, inconsistencies in measurement limit comparability across studies. The findings highlight the importance of family dynamics and early treatment response, supporting the need for more standardized research and individually tailored approaches.