BACKGROUND: Pre-marital sexual debut increase the risk of sexually transmitted infections (STIs) including HIV/AIDS and unwanted pregnancy. It may also affect their school performance and completion rate. In spite of this fact, number of unmarried female students who started sexual debut is increasing from time to time. However, information on the extent of pre-marital sexual debut and associated factors were not well studied and documented in the study area where pre-marital sexual debut is largely condemned. Therefore this study was conducted to assess the magnitude and associated factors of pre-marital sexual debut. METHODS: School based cross-sectional survey was conducted from May 10-13/2012. A total of 1123 unmarried high school female students were selected by multi- stage sampling technique. Data were collected using structured, self administered questionnaire. Descriptive statistics, binary and multivariable logistic regression analyses were used to identify factors associated with pre-marital sexual debut. RESULTS: Among unmarried high school female students 30.8% reported pre-marital sexual debut. The major associated factors were frequent watching of pornographic video [AOR = 10.15, 95% CI: (6.63, 15.53)], peer pressure [AOR = 2.98, 95% CI: (1.57, 5.67)] and chewing khat [AOR = 8.99, 95% CI: (3.84, 21.06)]. CONCLUSION: Significant proportion of unmarried high school female students have started pre-marital sexual debut. The finding suggests the need for communicating and supporting school students to help them make informed and safer decisions on their sexual behavior. Therefore, Bahir dar city administration health and education bureau should design persistent and effective health education to decrease pre-marital sexual debut in unmarried female students.