This study aimed to identify the association of perceived social norms with intention of early marriage practice among unmarried adolescent girls (13-17 years) in Kersa Demographic and Health Surveillance System. A community-based cross-sectional study was conducted during September--November 2019. Adolescent girls' marriage intention was assessed using social norm measuring tools. The log-binomial model was used to identify factors associated with girls' intention. Adjusted prevalence ratio along with 95% CIs was executed to determine the association of main independent variables with girl's intention. A total of 859 adolescent girls participated in the study. The prevalence of intention to marry before the age of 18 years among adolescent girls was 44.47% (95% CI [0.41%, 0.48%]). The intention for an early marriage was higher among those with positive attitude toward early marriage, in those who believe that most people in their reference group conform to early marriage norms (empirical expectation), in those who believe that most people in their reference group expect them to conform to early marriage norms (normative expectation) and who fear sanction related to early marriage practices. Moreover, residence in urban areas and high educational level by girls and their parents was associated with reduced prevalence ratio. There is strong social norm in the community that promotes intention for an early marriage among adolescent girls in the study area. Hence, interventions that aim to reduce early marriage need to address local social norms.