Abstract: Emerging research indicates that yoga is a promising adjunct to psychological trauma treatment. The current pilot study examined the associations between psychophysiological stress, diaphragmatic breathing (DB), and a trauma-sensitive yoga (TSY) regimen developed specifically for trauma-exposed service members in alignment with recent calls for precision in reporting therapeutic yoga protocols. Participants were 31 service members enrolled in a trauma-focused intensive outpatient program (IOP). Service members participated in a brief diaphragmatic breathing (DB) session using the Breathe2Relax (B2R) app followed by the TSY session. Heart rate (HR) and perceived stress were measured at baseline and after both the DB practice and the TSY session. We assessed Yoga and DB expectancies at baseline and post TSY. Participants also rated the acceptability and usability of the B2R app. Results of linear mixed effects regression models showed decreases in HR and perceived stress, compared to baseline, following DB (HR, b = -8.68, CI 95% = -13.34, -4.02; perceived stress, b = -1.77, CI 95% = -2.35, -1.18) and TSY (HR, b = -12.44, CI 95% = -17.15, -7.73; perceived stress b = -3.69, CI 95% = -4.29, -3.08). Higher levels of expectancies, compared to lower levels, related to stronger decreases in HR and perceived stress, particularly after TSY. Overall, participants rated the B2R usability as high; virtually all participants reported that "most would learn to use the app quickly," and 76.6% reported that they would use it frequently.