The problem of risky sexual behaviour has been a major concern for researchers, educationists and health professionals in both developing and developed nations. The reasons for these concerns are unconnected with associated negative outcomes. This study examined the influence of online social networking adoption on sexual risk behaviour among in-school adolescents. Three hundred and fifty-six students were randomly selected from ten day secondary schools in Ado-Odo Ota Local Government Area of Ogun State Nigeria. They included six public and four private schools. Only Senior Secondary two and three students participated in the study. There were 182 (51%) males and 174 (49%) females with mean age of 16.4years. The rate of online social networking adoption ranged from 3 hours per day to 5hours with the mean of 4.2hours. There is no significant difference in the rate of adoption of male and female students with mean of 4.2hrs for males and 4.19hrs for females. The rate of online social networking adoption was not significantly different based on age (t=0.06) and class (t=1.04) of the students. However, those that ever had sexual intercourse positively correlated with the rate of online social networking adoption (r=0.68, p=0.002). There was significant negative correlation between the age of first sexual intercourse and rate of online social networking adoption(r=-0.74, p= 0.005) while the number of sexual partners was significant positively correlated with the rate of online social networking adoption (r= 0.64, p= 0.004). The use of condom during the last sexual intercourse was negatively correlated with the rate of online social networking adoption(r=-0.55, p= 0.003). It was concluded that the rate of online social networking adoption has significant influence on sexual risk behaviour of in-school adolescents. It was recommended that parents should control the activities of their children and wards on online social networking.