Background and Objectives: Medical students represent the ideal target group for promoting mental health and mental wellbeing, being exposed to specific risk factors, such as the content of medical training, the exposure to sickness and death, and a stressful academic routine. Medical students report high levels of cynicism and emotional exhaustion, which represent two of the essential features of burnout syndrome. In this systematic review, studies assessing the levels of burnout among medical students through validated tools worldwide were analyzed. Materials and Methods: A systematic review has been performed in order to identify studies: (1) focusing on samples of medical students; (2) evaluating burnout syndrome using validated tools; (3) providing prevalence data on burnout; and (4) written in English. Results: Out of the 5547 papers initially obtained, 64 were finally included in the analysis. The sample sizes ranged from 51 to 2682 participants. Almost all studies had a cross-sectional design; the Maslach Burnout Inventory and its related versions were the most frequently used assessment tools. The prevalence of burnout, which was stratified based on gender and academic stage, ranged from 5.6 to 88%. Burnout was mostly predicted by thoughts of stopping medical education, negative life events, lack of support, dissatisfaction, and poor motivation. Conclusions: The prevalence of burnout syndrome in medical students is quite heterogeneous, reaching a peak of 88% in some countries. However, several predictors have been identified, including negative life events or poor motivation. These findings highlight the need to develop preventive interventions targeting the future generation of medical doctors, in order to improve their coping strategies and resilience styles.