Introduction: Quality of life (QoL) is a multidimensional measure of adolescent well-being encompassing physical, psychological, social and environmental domains. Understanding factors associated with QoL is essential for informing pediatric and school health interventions. However, district-level evidence from Sri Lanka remains limited. Objectives: This study was conducted to assess quality of life and its associated factors among adolescent schoolchildren in Galle district, Sri Lanka. Methodology: A school-based cross-sectional study was conducted among 876 adolescent schoolchildren aged 16-19 years selected using multistage cluster sampling. Quality of life was assessed using the validated, Sinhala version of World Health Organization Quality of Life-Brief questionnaire. Data on sleep duration, physical activity and social support were collected using a judgmentally validated, self-developed questionnaire. Independent sample t-test was used to compare mean overall QoL scores between groups. Results: Mean age (SD) was 16.27 (0.58) years. The mean general health score was 7 .67 ± 1.46. Domain-specific mean scores were: Physical 27 .81 ± 3.68, Psychological 22.21 ± 3.60, Social Relationships 10.86 ± 1.66, and Environmental 29.62 ± 4.56. Adolescents with inadequate sleep, low physical activity levels and lack of social support demonstrated significantly lower overall QoL scores (p < 0.05). There was no significant difference in mean QOL score between males and females. Conclusions: Psychosocial and lifestyle factors are associated with adolescent quality of life. Targeted interventions focusing on sleep hygiene, physical activity promotion and strengthening social support systems should be integrated into adolescent school health services to enhance adolescent quality of life.