This cross-sectional study examined how Chinese adolescents' belief in a just world (BJW) varied across social class and age. A sample of 2,748 adolescents aged 10 to 18 years (47.2% female; 99.1% Han ethnicity) from diverse socioeconomic backgrounds in China completed the Belief in a Just World Scale and the Socioeconomic Status Survey. Adolescents from higher social-class backgrounds were found to have higher general and personal BJW compared to their peers from lower social-class backgrounds, with the difference being more pronounced for personal BJW. Additionally, SES moderated age-related changes in BJW: for adolescents from middle- or higher-class backgrounds, BJW decreased with age, whereas for those from lower-class backgrounds, BJW remained relatively stable across adolescence. These findings suggest that BJW functions as a psychological defense mechanism, shaped by an individual's perceived access to fairness and opportunity within their social environment.