Introductionadolescents are at high risk of engaging in risky sexual behaviours that could predispose them to several health problems. Cognizant of these facts, studies addressing the root causes for risky sexual behaviours in risk areas like Shewa Robit are mandatory. Therefore, the study was conducted to assess risky sexual behaviours and its determinants in Shewa Robit town, Northeast Ethiopia.Methodsan institutional-based cross-sectional study was conducted among 349 students using quantitative and qualitative approaches. For the quantitative study, data entry and analysis was done by using Epidata-3.1 and statistical package for social sciences (SPSS)-20, respectively. Binary and multiple logistic regression analysis was used to identify factors associated with risky sexual practices. Odds ratio with 95% confidence interval (CI) was used for measuring the strength of association. Variables with P-value of <0.05 were considered statistically significant. Collected data from the qualitative study was debriefed, categorized, coded and common themes were generated manually. Finally, the findings were triangulated with quantitative findings.Resultsa total of 338 respondents involved in the study. Overall, 168 (49.7%) of respondents have had risky sexual behaviour. Alcohol consumption (Adjusted odd ration (AOR)=5.01, 95%CI; 2.12-11.79), peer pressure (AOR=5.82, 95%CI; 2.97-11.41), grade level (AOR=5.82, 95%CI; 2.06-16.45), and family size (AOR=5.19, 95%CI; 2.24-12.01) were significantly associated with risky sexual behaviour.Conclusionrisky sexual behaviour in the study area was high. Several families and school-level factors were found to be major determinants of risky sexual behaviours. Public health interventions focusing on adolescent sexual health should be targeted to save those risky adolescents.