Background and objective While smartphones offer various benefits, addiction to them among young people poses a serious problem for parents and educators globally. Several studies have tried to assess the impact of the excessive use of/addiction to smartphoneson students' overall academic performance. While a few studies have found a positive impact, most have observed a negative impact, mainly in the form of deterioration of mental and physical healthand academic performance of students. In light of this, we conducted this study in three medical colleges in Karnataka, India with the objectives of determining the effect of smartphone addiction on the academic performance of undergraduate medical students and to identify the risk factors associated with it. Material and methods This cross-sectional study, conducted between July 2022 and October 2023, involved undergraduate medical students from Yenepoya Medical College, Subbaiah Institute of Medical Sciences, and Adichunchanagiri Institute of Medical Science. A total of 481 students were enrolled by stratified random sampling. Data were collected with the help of Google Forms by using a pre-tested questionnaire. Mobile phone addiction was assessed by using the smartphone addiction scale-short version (SAS-SV). Results Among 481 students, 211 (43.9%)were found to be mobile phone addicts. An independent t-test showed that academic performance was negatively affected in students who were mobile phone addicts. Multivariate logistic regression analysis revealed several factors significantly associated with mobile phone addiction, including male gender, regular mobile gaming, seeking recognition/popularity through social media, frequent smartphone checking, perceiving smartphone use as more attractive than other activities, limited co-curricular activities, and increased smartphone use during periods of depression or anxiety. Conclusions Based on our findings, smartphone addiction has a negative impact on the academic performance of students. To mitigate this issue, educational institutions should integrate efforts to tackle smartphone addictioninto their curricula and prioritize addressing the identified risk factors.