Objective.-Scalable, accessible forms of behavioral therapy for migraine prevention are needed. We assessed the feasibility and acceptability of progressive muscle relaxation (PMR) delivered by a smartphone application (app) in the Primary Care setting. Methods.-This pilot study was a non-blinded, randomized, parallel-arm controlled trial of adults with migraine and 4+ headache days/month. Eligible participants spoke English and owned a smartphone. All participants were given the RELAXaHEAD app which includes an electronic headache diary. Participants were randomized to receive 1 of the 2 versions of the app-one with PMR and the other without PMR. The primary outcomes were measures of feasibility (adherence to the intervention and diary entries during the 90-day interval) and acceptability (satisfaction levels). We conducted exploratory analyses to determine whether there was a change in Migraine Disability Assessment Scale (MIDAS) scores or a change in headache days. Results.-Of 139 participants (77 PMR, 62 control), 116 (83%) were female, mean age was 41.7 ± 12.8 years. Most patients 108/139 (78%) had moderate-severe disability. Using a 1-5 Likert scale, participants found the app easy to use (mean 4.2 ± 0.7) and stated that they would be happy to engage in the PMR intervention again (mean 4.3 ± 0.6). For the first 6 weeks, participants practiced PMR 2-4 days/week. Mean per session duration was 11.1 ± 8.3 minutes. Relative to the diaryonly group, the PMR group showed a greater non-significant decline in mean MIDAS scores (−8.7 vs −22.7, P = .100) corresponding to a small-moderate mean effect size (Cohen's d = 0.38). Conclusion.-Smartphone-delivered PMR may be an acceptable, accessible form of therapy for migraine. Mean effects show a small-moderate mean effect size in disability scores.