Premarital sex is having sex before marriage. The research was conducted on girls at Yismala secondary school. Multi phase or iterative design was applied. Samples were selected from grade 11 and 12 students. From the 242 population 191 sample students were selected from grade 11 students. To study psychosocial consequences of premarital sex 10 students were selected from grade 12. 0ne hundred ninety one grade 11 students were participated in the study by using stratified sampling technique by using grade level as a stratum. Convenience sampling technique was employed for interview part and two forms of data instruments (questionnaire and interview) were used. The questionnaire was assessed through pilot test to check its reliability. The result shows that among the variables entered to predict whether a girl would engage in premarital sex, self-esteem, maternal monitoring, and peer influence turned significant predictors. While self-esteem and maternal monitoring negatively predict, peer influence positively predicts girls’ likelihood to engage in premarital sex. The qualitative finding has shown that emotional disturbance, inferiority, self- withdrawal, depression, anxiety, suicide, hopelessness, unplanned pregnancy, unsafe abortion, fistula, sexually transmitted diseases, dropout from school, and social withdrawal are the psychosocial consequences of premarital sex. Based on findings of the study, it was concluded that self-esteem, maternal monitoring and peer influence are important precursors of premarital sex. Premarital sex has several undesirable consequences ranging from physical health to mental health. Based on the results of the study it was recommended among others that sex education should be integrated in to secondary school curriculum and there is an urgent need of designing an intervention on addressing the high level of premarital sex in the study area.