Introduction: Several studies in Nigeria have reported high rates of risky sexual activities among adolescents. This study, therefore, aimed to describe the sexual risk behaviors among adolescents attending secondary schools in a Southwestern State in Nigeria. Materials and Methods: This was a descriptive cross-sectional study, carried out among in-school adolescents in Osun State, Southwestern Nigeria. The multi-stage sampling technique was used to select the respondents, and data were collected with pre-tested, semi-structured questionnaires which were self-administered and supervised by trained research assistants. A total of 815 questionnaires was administered, but only 786 (96.4%) were properly filled and analyzed using Statistical Package for Social Sciences version 16. Results: Many of the respondents (406, 51.7%) were early adolescents, while the remaining 380 (48.3%) were late adolescents with a mean age of 14.2 ± 2.2 years. One hundred and eight (22.9%) were sexually exposed with a mean age of sexual debut of 12.7 ± 2.7 years. Of the 180 that were sexually exposed, 134 (74.4%) had sex in the month preceding the study, 40 (22.2%) had paid for or demanded for payment for sex before, and only 48 (26.7%) used condom for their last sexual intercourse. Only 2 (0.3%) respondents reported having had sexually transmitted infections (STI), but, 128 (16.3%) had had symptoms suggestive of STI before. Conclusion: There was a high level of sexual risk behaviors among the respondents with 22.9% of them being sexually exposed with a mean age at sexual debut of 12.7 ± 2.7 years. Public health interventions to prevent human immunodeficiency virus/acquired immunodeficiency syndrome should target adolescents in their early ages before they initiate sexual activity.