Objectives To determine association between physical activity and hours of physical activity and stress levels in medical students  Methods A cross sectional study was conducted at a private medical college in Karachi, during April and May 2015. Convenience sampling was used to identify the study participants. The subjects were medical students from year one through five. We interviewed 235 students. A pre-tested, self-administered three-part questionnaire was used to collect the data. The first part of the questionnaire inquired about the subjects’ demographics and exercise habits, the second part of the questionnaire was adapted from the Perceived Stress Scale (PSS) to determine stress scores within participants, and the third part of the questionnaire asked about other stress relieving factors. Results According to the PSS, 30.3% of the participants were moderately stressed. The mean PSS was found to be 16.95 ± 5.72. 2A significant correlation was found between physical activity and stress levels; those involved in physical activity were 52 percent less likely to be moderately stressed (OR 0.48, p-value = 0.015) as compared to those who had not partaken in physical activity. Furthermore, studentswho performed physical activity for >4 hours per week had a lower mean PSS score (16.2) when compared to those who exercised for <2 hours per week. (18.2). Post-hoc statistical testing revealed the difference to be significant (p=0.028). Conclusion Based on the findings of our study, we conclude that physical activity of any form is associated with significantly decreased stress levels in medical students.