The present study piloted a brief, single-session behavioral intervention to prevent stress-related illnesses in college students. Participants were recruited at a large public university ( N =  213). A total of 132 participants completed in-person research sessions and were randomly assigned to one of three conditions (two control and one intervention), and another 81 participants completed measures online (online control) without any face-to-face contact. The intervention consisted of brief, one-on-one diaphragmatic breathing training, delivered by trained research assistants. Data were collected at baseline and 2-week follow-up. Participants receiving the intervention provided positive ratings of intervention acceptability and reported utilizing diaphragmatic breathing during the 2-week period. They also reported significant improvements in perceived stress, anxiety, and depressive symptoms. When compared with the control conditions, however, the Intervention condition produced superior improvement in depressive symptoms only. Results suggest that diaphragmatic breathing training can be offered by trained peers to college students to help reduce stress-related symptoms.