Persistent post-traumatic headache (PPTH) is often the most common injury post mild traumatic brain injury (mTBI), reported by 47%-95% of patients. Progressive muscle relaxation (PMR) has level A evidence in preventing migraine and tension headaches. However, research on this behavioral therapy for PPTH, let alone smartphone-delivered, is limited. We performed a single-arm study of prospective patients calling our Concussion Center between June 2017-July 2018. Inclusion criteria were that subjects had to meet ICHD-3 criteria for PPTH secondary to mTBI, have four or more headache days a month, be age 18-85 and 3-12 months post injury, own a smartphone and not tried headache behavioral therapy within the year. We recorded baseline headache and neuropsychiatric data. Using the RELAXaHEAD smartphone application, which has a headache diary and PMR audio files, participants were instructed to record headache symptoms and practice 20 minutes of PMR daily. There were three monthly follow-up assessments. There were 49 subjects enrolled. Basic demographics were: 33 (67%) female with mean age 40.1±14.6 [20,75]. Of the 49 subjects, 15 (31%) had pre-existing headaches. In 11 (22%) subjects, mTBI was sports-related. Subjects reported 17.7±9.3 [4,31] headache days in the month before enrollment, and 49 (100%) experienced over three concussion symptoms. Participants inputted data in the RELAXaHEAD app on average 18.3±12.0 days [0,31] the first month. Number of participants who did PMR over 4 times/week was 12 (24.5%) the first month, 9 (22.5 %) the second month, and 6 (15%) the third month. After three months, 17 (42.5 %) participants continued doing PMR. Participants cited time constraints, forgetfulness, application glitches and repetitiveness as obstacles to practicing PMR. It is feasible to get PPTH subjects to practice behavioral therapy through low-cost smartphone-based PMR two times weekly. Future work will assess efficacy and examine how to optimize barriers to PMR.