BACKGROUND: Posttraumatic stress disorder (PTSD) disproportionally affects low-income, racial and ethnic minoritized communities, where prevalence is high, yet access to evidence-based treatments (EBTs) is low. As such, there is a need to identify effective, feasible, and scalable interventions for PTSD. Stepped care approaches that include brief, low-intensity treatments are one approach to improving access yet have not been developed for adults with PTSD. Our study aims to test the effectiveness of a step one PTSD treatment in primary care while gathering information on implementation to maximize sustainability in the setting. METHODS: This study will be conducted in integrated primary care in the largest safety net hospital in New England using a hybrid type 1 effectiveness-implementation design. Eligible trial participants are adult primary care patients who meet full or subthreshold criteria for PTSD. Interventions include Brief clinician-administered Skills Training in Affective and Interpersonal Regulation (Brief STAIR) versus web-administered STAIR (webSTAIR) during a 15-week active treatment period. Participants complete assessments at baseline (pre-treatment), 15 weeks (post-treatment), and 9 months (follow-up) post-randomization. We will assess feasibility and acceptability post-trial using surveys and interviews with patients, study therapists, and other key informants, and will assess the preliminary effectiveness of interventions in terms of PTSD symptom change and functioning. CONCLUSION: This study will provide evidence for the feasibility, acceptability, and preliminary effectiveness of brief, low-intensity interventions in safety net integrated primary care, with the aim of including these interventions in a future stepped care approach to PTSD treatment. CLINICAL TRIAL NUMBER: NCT04937504.