Objective: This study aimed to examine the effects of yoga and physical fitness exercises on stress and the underlying mechanisms. Participants: Healthy undergraduates from four yoga and four fitness classes participated in Study 1 (n = 191) and Study 2 (n = 143), respectively (in 2017 Fall). Methods: Study 1 evaluated the immediate effect (a 60-minute practice) while Study 2 evaluated the durable effect (a 12-week intervention). Results: Results showed that immediate stress reduction was more salient in the yoga group than that in the fitness group in Study 1. Yoga group had a greater increase in mindfulness, which predicted stress reduction. Similar observations were made in Study 2 showing the durable effect of yoga on stress reduction through mindfulness. Conclusions: Yoga intervention is better than fitness exercises in helping undergraduates cultivate mindfulness and reduce stress. These findings may guide future interventions in stress management in college students.