Abstract Objectives: This study aims to investigate predictors of insomnia and their prevalence among medical students from different levels, as well as the impact of insomnia on academic performance. Methods: This multicenter cross-sectional study was conducted on medical students of different levels at 37 medical colleges in Saudi Arabia from March 2020 to June 2020. An online survey was used for data collection. The survey included questions on the demographics of students and two scales: the Insomnia Severity Index (ISI) and the Pittsburgh Sleep Quality Index (PSQI). The Statistical Package for the Social Sciences Program was used to analyze the data, and demographics were represented using descriptive statistics. Associations were tested using either the Chi-square test, Pearson correlation, or Spearman’s correlation based on the types of variables. P <0.05 was considered significant. Results: The study included 2413 medical students; the mean ± standard deviation of the ISI and PSQI was 11.6 ± 6.2 and 8.4 ± 3.7, respectively. The ISI results showed that only 27% had no clinical insomnia, while 39% had subthreshold insomnia. The ISI was negatively and significantly associated with exercise (r = −0.49, P = 0.016), meals (r = −0.088, P = 0.0001), and drinking water (r = −0.057, P = 0.005). The negative predictors of poor sleep quality included study year (B = −0.106, P < 0.05) and water intake (B = −0.568, P < 0.01). Conclusion: A few proportions of medical students complained of moderate insomnia, and a lower proportion suffered severe insomnia. Lower academic performance was associated with insomnia and poor sleep quality. Various predictors of insomnia were identified; however, these predictors can be modified.