This study compared deep diaphragmatic breathing (DB) versus paced breathing and progressive muscle relaxation (PMR) on indices of physiological stress reactivity in 76 varsity athletes. Athletes were trained in paced breathing, and then randomized to either DB or PMR, and underwent a cognitive stressor task. DB resulted in significantly higher tidal volume compared to PMR and paced breathing. DB also resulted in lower heart rate than PMR. Finally, DB resulted in significantly higher heart rate variability (HRV) than PMR, but not compared to simple paced breathing. Both groups reported feeling significantly more relaxed after the intervention than after paced breathing. The DB group reported a trend toward greater relaxation than the PMR group. Slow DB is an easy to teach tool that may give athletes an edge on successfully managing stress reactivity, whether they are preparing for a test or for competition.