Introduction Adolescence is a developmental period characterised by various opportunities including the social media. Adolescence is also embedded with several challenges that are dangerous to sexual health. In Nigeria, research focusing on the effects of social media on adolescents’ sexual behaviour (SB) is very rare. This study was therefore designed to determine the use social media (SM) and its effects on SB of in-school adolescents in Ibadan, Nigeria.

Methods A cross-sectional study was carried out among randomly selected 194 senior secondary students in four secondary schools in Ibadan. The questionnaire explored the socio-demographic data, SB, frequency of use of SM, and activities on SM. Data was analysed using descriptive statistics and Chi-square at 0.05 level of significance.

Results The mean age of the respondents was 14.7±1.2 and 50.3% were females. Some (15.2%) reported ever experienced sexual intercourse. The mean age of sexual debut was 12.2±2.8. Most (88.0%) reported the use SM, the highest used social medium was Facebook (71.2%) followed by Whatsapp (55.0%) while the most frequently used was Whatsapp followed by facebook; 44.7% and 25.4% reported everyday use respectively. Activities reported on SM included reading of news (60.7%), download/listen to music (62.8%), watch/download pornographic pictures (5.2%), watch/download pornographic videos (8.4%), search for sexual partners (16.8%), share pornographic pictures (4.2%) and videos (4.7%). Significantly, more of those who had ever experienced sexual intercourse watched/downloaded pornographic pictures (24.1% vs 1.9%) and videos (37.9% vs 3.1%), and search for sexual partners (58.6% vs 9.3%) using the SM.

Conclusion A number of the adolescents use social-media and many engage in activities that could be risky to their sexual life on these media especially those who have ever experienced sexual intercourse. Intervention to ensure safe use of the social-media should be carried out among these young persons.