Stress contributes to psychological and physiological strain, yet established interventions (e.g., CBT, mindfulness, peer support) can be costly or impractical at scale. Diaphragmatic breathing may offer a brief, low-cost alternative. This study tested whether a short, self-guided diaphragmatic breathing intervention reduces stress, anxiety, and depressive symptoms. Ninety-nine participants (83.8% female; M = 24.34, SD = 10.56) were recruited via social media and the University of Chichester participation scheme and randomly assigned to an active breathing group or waitlist control. The active group completed five minutes of self-guided diaphragmatic breathing daily for seven days. Participants completed pre- and post-intervention measures (PSS, GAD-7, PHQ-9). One-way ANOVAs on pre–post difference scores showed no significant group differences in stress, anxiety, or depression. Participants reported moderate motivation and perceived usefulness. Findings suggest brief self-guided breathing may be insufficient without longer duration or instructor support; further mixed-methods research is recommended.