This pilot open trial examined the efficacy of attachment-based family therapy (ABFT) for Israeli sexual and gender minority (SGM) young adults and their persistently nonaccepting parents. Thirty families received up to 26 weeks of treatment, with parental rejection, parental acceptance, and young adults’ attachment avoidance and attachment anxiety assessed at baseline, 8, 16, 24, and 36 weeks (three months post-treatment). Analyses using multilevel growth models revealed that both young adults and their mothers independently reported increases in mothers’ acceptance of their young adult’s same-sex orientation or noncisgender identity. In addition, young adults reported decreases in both parents’ levels of rejection. Also, mothers, but not fathers, reported decreases in their own level of rejection. Finally, young adults reported a decrease in attachment avoidance in their relationships with both mothers and fathers, but not a decrease in attachment anxiety. Importantly, these treatment gains were maintained three months after the end of treatment. Together, these results suggest that ABFT-SGM, a manualized, affirmative, experiential, family-based treatment, may be effective in reducing long-standing parental rejection, promoting parental acceptance, and improving the quality of LGBTQ+ young adults’ relationships with their parents. These findings are encouraging in light of the urgent need for efficacious interventions to reduce family generated minority stress and promote safer, more supportive environments for sexual and gender minority people.