The study examined the dimensions and levels of internet addiction among youths in a Nigerian university. Based on the students’ population of 22,313 undergraduates of Obafemi Awolowo University, the study sampled 1,448 students across all 13 faculties using a proportionate sampling technique. A questionnaire tagged “Internet Addiction and Online Sexual Behaviour of Students Questionnaire (IAOSBQ)” was used to collect data for the study. The study found that partnered-arousal online sexual conduct (50.6%) was the most common dimension of online sexual behaviour exhibited, whereas solitary-arousal behaviour (17.8%) was the least common. It also showed that 14.1% exhibited a severe level of internet addiction, with only 5.3% exhibiting normal internet use. Students who exhibited a high level of online sexual behaviour showed a much stronger likelihood of experiencing internet addiction (Β = 5.898, p < 0.001, 95%CI [4.604 -7.550]). The study concluded that excessive involvement in online sexual activity is a major determinant of excessive internet use among university youths. Relevant recommendations and appropriate policy implications were drawn accordingly.