Objective: Internet addiction among university undergraduates may deeply impact their academic lives. This study investigated the association between academic fatigue, academic engagement, academic performance, gender, and internet addiction and how the variables jointly and relatively predict internet addiction among undergraduates in southwestern universities in Nigeria. Method: The correlational research type of the non-experimental research design was adopted for this study. A multi-stage sampling procedure was used to select 1,157 students. The main variables were the Internet Addiction Test (r = 0.82) and the Academic Fatigue Questionnaire (r = 0.85; both adapted), as well as the Academic Engagement Scale (r = 0.93; researcher developed) and cumulative grade point average. We analyzed the data using frequency, percentages, correlation, and multiple regression. Results: Undergraduate males are significantly more addicted to the internet than females. Academic fatigue, academic engagement, male gender, and internet addiction had a positive significant relationship, while academic performance did not. The multiple association among academic fatigue, engagement, male gender, and internet addiction was 0.6, while the variance accounted for by the predictors on internet addiction was 35.2%. Further, academic fatigue ([beta] = 0.346, t = 14.042, p <0.01), academic engagement ([beta] = 0.394, t = 15.741, p<0.01), and male gender ([beta] = 0.191, t = 7.609, p <0.01) are the potent predictors of internet addiction among undergraduates. Conclusion: Academic fatigue, academic engagement, and gender (specifically male) are leading causes of increased internet addiction among undergraduates. This study contributes to the extant literature by providing evidence that academic performance or success does not necessarily predict internet addiction among university students. Nevertheless, undergraduates should be helped to moderate internet usage.