Background: Sedentary behavior is associated with various adverse health outcomes. Medical students often experience high academic demands, leading to increased sedentary time. This study aimed to evaluate the effectiveness of a mobile app providing real-time feedback in reducing total sedentary time and prolonged sedentary bouts and in promoting physical activity among medical students. Methods: Seventy-seven medical students from Jazan University (mean age: 21.4 years; range: 20–25 years) participated in this study. Participants were assigned to either the control group (n = 40) or the intervention group (n = 37). The intervention group received real-time vibration feedback via a mobile app, prompting movement every 30 min of sedentary time, while the control group received no intervention. Sedentary behavior and physical activity levels were assessed using the Activities Completed Over Time in 24 h. Paired t-tests were conducted to examine within-group changes, and a two-way ANOVA was used to assess the interaction effect of time and group on sedentary time and physical activity. Results: After six weeks, the intervention group showed significant within-group reductions in their total sedentary time of 1.82 h (p = 0.01) and prolonged sedentary bouts of 1.91 h (p = 0.001), while the control group had no significant changes. Physical activity levels did not significantly change within either group. The two-way ANOVA revealed that there was no significant change over time between the two groups in their total sedentary time F (1, 75) = 1.590, p = 0.21, prolonged sedentary bouts F (1, 75) = 3.499, p = 0.06, or physical activity F (1, 75) = 0.565, p = 0.45. Conclusions: Real-time feedback from mobile apps resulted in significant within-group reductions in total and prolonged sedentary time among medical students in the intervention group. Low-cost mobile apps providing real-time feedback may be an effective intervention for reducing sedentary behavior among medical students, potentially improving their health and well-being.