Introduction: Common mental illness is a group of mental disorders that include depression, anxiety, and somatoform disorders. The World Health Organization estimated that common mental illness accounts for 30% of non-fatal disease burden worldwide. Mentally distressed individuals manifest with different level of mental disorder symptoms. There is limited school based research on adolescent mental health especially in developing countries like Ethiopia, particularly in Hawassa city, Southern Ethiopia. Objectives: To assess the prevalence of common mental illness and associated factors among Hawassa city high school students, Hawassa, Southern Ethiopia. Methods: Institution based cross-sectional study design was implemented between November and December 2020. Multistage sampling technique was used to select 634 students to collect the required data. Self-Reported Questionnaire (SRQ-20) was used to assess common mental illness. Data were coded and double entry undertaken using EpiData version 3.1 and the data was exported to Statistical Package for Social Sciences (SPSS) version 26 for cleaning and analysis. Bivariable analysis was undertaken and variables with P-value <0.25 in the bivariable analysis were entered into multivariable regression model. Multivariable logistic regression was used to measure associated factors with common mental illness after controlling for potential confounding factors. Finally, the result was presented using odds ratio with 95% confidence interval and P value <0.05 was considered as cut of point to declare significant association. Result: The overall prevalence of common mental illness was 20.7%. After adjusting for other variables age 15-19 years [AOR=0.51, 95% CI: (0.27, 0.97)], female sex[AOR=1.79, 95% CI: (1.10, 2.39)], grade 9 [AOR=0.41, 95% CI: (0.22, 0.76)], grade 10 [AOR=0.46, 95% CI: (0.23, 0.82)], chew khat currently [AOR=2.46, 95% CI: (1.07, 5.65)], poor social support [AOR=2.34, 95% CI: (1.27, 4.32)]and had chronic medical illness[AOR=3.16, 95% CI: (1.64, 6.09)] were independent predictors of common mental illness. Conclusion: The prevalence of common mental illness among high school students in the study area is high. Older age, female sex, higher grade, khat chewing, poor social support and having medical illness were risk factors for common mental illness. Guidance and counseling, specially for high graders and females, strengthening student club (anti-drug) and improving student, parent and teachers communication were recommended.