There are many social and cultural factors affecting the sexual knowledge of adolescents. This study measured the sexual health knowledge level of adolescents and identified its association with role of parents, friends and school environment in adolescent girls in Riyadh, Saudi Arabia.Four hundred and nineteen Saudi female students belonging to intermediate and secondary grades were randomly selected from four public and private girl schools. 255 (69.8%) students were ≤15 years and 164 (39.2%) were >15 years. A self-administered structured questionnaire comprising of socio-demographic information, role of parents and teachers, availability of school curriculum on sexual health was used. Sexual health knowledge was assessed through questions on identification of physical changes during puberty for ≤15 years and separate questions on sexually transmitted infections for >15 years.54% of ≤15 years and 70.7% of >15 years had poor sexual health knowledge. Multivariate analysis found determinants for poor sexual health education in ≤15 years are: lower education level of both parents (OR 10.87; 95% CI 2.44-48.38), second birth order or more (OR 2.32; 95% CI 1.24-4.33) and absence of school curriculum on sexual health (OR 0.56; 95% CI 0.33-0.95). Determinants for >15 years of age are : mothers with low literacy (OR 3.08, 95% CI 1.42-6.71), as for sources of poor sexual knowledge : parents (OR 10.10; 95% CI 2.70-37.74), schools (OR 6.95; 95% CI 1.95-24.78) maids (OR 4.57; 95% CI 1.26-16.59) and media (OR 5.12; 95% CI 1.29-20.07) were statistically significant factors.Government agencies with collaboration of all stake holders should develop policies and programs for implementing and evaluating integrated and comprehensive sexual educational programs for adolescents in Saudi Arabia.