Background: Premarital sex may result in sexually transmitted infections (STIs) and unwanted pregnancy. Published reports on the prevalence premarital in private colleges in Ethiopia are limited. This study aimed at assessing the prevalence of premarital and risky behaviours among students of Ayer Tena Health Science and Business College, Addis Ababa, Ethiopia.Methods: A cross-sectional study was conducted among of Ayer Tena Health Science and Business College, Addis Ababa, Ethiopia. A semi-structured questionnaire was used to collect data from 390 students that were selected by systematic random sampling. Data analysis was done using SPSS version 23. Associations between variables were determined using chi-square test.Results: The study participants consisted of 81% females, 73.7% Christians, 58% business students, 56.8 % degree students, 62.1% regular students and 49.1% fresh students. We found that 23.3% of the students had exprienced premarital sex, of which 43.5% had sex in the last 12 months, 14.3% were homosexuals, and 20% started sex before 18. About 68.2% never used condom at the first sex and 60% had history of having sex without condom. About 51.3% did not plan for the first sex. Nearly 34% had multiple sexual partners in their life time and 15.5% had multiple current sexual partners. From 334 respondents, 14.7% said that oral contraceptive pills prevent sexually STIs. Nearly 59 % (47/80) of the study subjects did not have a practice of requesting a new sexual partner for STIs test result. Many students had a history of watching pornography (17.3%), drinking alcohol (17.3%) and taking drugs (3%). Among 65 female students, 20% had a history of pregnancy, of which 46.2% have done abortion. Among those who practiced sexual intercourse, 5.9% had history of STIs, including HIV. Premarital sex was common among males, Christians, health students, non-regular students, degree students, and among those who never attended religious services, live alone, have income, watch pornography, believe that oral contraceptive pills prevent STIs, and take alcohol.Conclusion: The prevalence of premarital is high among the study participants. Risky behaviors and misconceptions are also common. Hence, it would be better to devise and implement effective intervention strategies to reverse the situation.