Background: Sleep problems and poor sleep quality are important issues for medical students. This study aimed to investigate the sleep patterns, measure the prevalence of poor sleep quality, and identify the predictors of poor sleep among medical students in King Khalid University (KKU), Saudi Arabia. Methods: This cross-sectional study enrolled 318 medical students during October– November, 2015. Participants were selected by convenience sampling and data were collected using self-administered questionnaires to obtain information regarding socio-demographic variables and indicators of sleep quality. Results: The overall mean sleep quality score was 6.79 with a standard deviation of 3.06. Poor sleep quality was reported by 74.2% students. Significantly high mean sleep quality scores (Pittsburgh Sleep Quality Index) were observed for students with very poor subjective sleep quality (mean = 10.50, SD = 2.58), least sleep efficiency (mean = 11.21, SD = 2.23), shorter sleep duration (mean = 7.83, SD = 2.88), sleep onset latency more than 30 minutes (mean = 7.82, SD = 2.53), sleeping after midnight (mean = 7.53, SD = 2.95), and use of sleep aiding medication (mean = 8.78, SD = 3.5). Significant differences were observed between good sleepers and poor sleepers regarding these sleep characteristics. Poor sleep was predicted by sleep behaviours such as going to sleep after midnight (AOR = 2.18, 95% CI: 1.20, 3.94) and sleep duration of less than seven hours (AOR = 7.49, 95% CI: 4.24, 13.22). Conclusion: Medical students of KKU have poor sleep quality. Longer sleep latency, going to sleep after midnight, and shorter sleep duration are important problems in this group.