Background and Objective: Globally, loneliness is considered a public health concern. Adolescents may be particularly vulnerable to feelings of loneliness because they face unique development challenges. This study investigates the overall changes in the prevalence of loneliness among the nationally representative study populations of 13–19-year-olds in Norway over the last three decades, and whether age, gender, self-rated health (SRH), and mental distress are associated with these changes. Methods: Data was used from the Norwegian population-based Young-HUNT survey in 1995–1997, 2006–2008 and 2017–2019 (N=25,245). Loneliness was measured by the question ‘Do you feel lonely?’ Mental health was assessed by Hopkins Symptom Checklist-5 for adolescents to measure anxiety and depression (cut-off ≥2). SRH was assessed by a single direct question ‘How is your health at the moment?’. Descriptive analyses were stratified by age, gender, SRH, and mental distress over survey year. Logistic regression and interaction analyses were used to analyze the associations and predictors of loneliness. Results: Prevalence of loneliness increased from 5.9% in 1995/97 to 10.2% in 2017/19. An increase of 8.9% in 1995/97 to 16.7% in 2017/19 was observed in girls 16-19 years old. Among mentally distressed adolescents, loneliness prevalence increased from 22.3% in 1995/97 to 32.8% in 2006/08 and lowered to 27% in 2017/19. Similar trends were seen in those with poor SRH. There was a statistically significant interaction effect between age and time point on loneliness. Conclusions: Results indicate increasing levels of adolescent loneliness over the last three decades in Norway corresponding with findings of increasing trends of poor mental health. This highlights the importance of developing interventions for awareness, social integration, and health promotion. Schools may be an especially ideal setting for intervention with the possibility to work on the psychosocial environment, social skills, parental education and early recognition of loneliness and mental health problems.