Background: Understanding young people's subjective well-being, such as how they experience equality, safety, and supportive environments, is particularly important for identifying possible preventive interventions in the school context. Aim: The aim of this study was to investigate similarities and differences between 15-year-old girls’ and boys’ subjective well-being, and its associations with psychosocial factors. Methods: The study was explorative and cross-sectional. A total of 195 adolescent girls and 116 boys in Grade 9 from four Swedish junior high schools completed a questionnaire on subjective well-being; emotional support opportunities; gender equality; experience of emotional, physical, and sexual violence in close relationships; insecurity due to harassment in the school environment or on social media; and pornography consumption. Results: Compared to boys, girls indicated worse subjective well-being and more harassment in school and on social media, and reported that boys received benefits based on gender. Boys reported higher pornography consumption compared with girls. Girls reporting decreased subjective well-being experienced more harassment on social media and in school, and more recent sexual violence. Among boys, with decreased subjective well-being, more psychological risks such as self-harm and suicidal thoughts was reported. Both girls and boys with decreased subjective well-being reported less availability of attachment and insecure close relations. Conclusion: The absence of a safe and supportive environment, such as feeling secure at school, and lack of close and trusting relationships are associated with 15-year-olds’ subjective well-being for the worse, regardless of gender. It is suggested that school personnel extend the relational possibilities in the school context.