["Scandinavian Journal of Psychology, Volume 67, Issue 2, Page 620-631, April 2026. ", "\nABSTRACT\nThis study aimed to expand knowledge on the roles of psychological inflexibility and school friendship closeness in adolescents' loneliness during the transition to upper secondary education. The participants were 885 Finnish adolescents (mean age = 15.74, SD = 0.37, 56% girls). Loneliness was measured twice: in Grade 9 and at the beginning of upper secondary school in 10th grade. Psychological inflexibility and the closeness of friendships in school were measured in Grade 9. Results showed that a high level of psychological inflexibility in Grade 9 was associated with higher loneliness at the beginning of upper secondary education and increased loneliness during the educational transition. Furthermore, girls' (but not boys') friendship closeness in school partly mediated the association between psychological inflexibility and loneliness. The results suggested that psychological inflexibility is a social risk factor for loneliness among adolescents transitioning to upper secondary school. Enhancing psychological flexibility skills could thus make an important contribution to the promotion of adolescent peer relationships and prevention of loneliness.\n"]