To widen treatment access for posttraumatic stress disorder (PTSD) in resource-constrained South Africa, we evaluated the feasibility and effectiveness of a counsellor-supported PTSD Coach mobile application (app) (PTSD Coach-CS) intervention on PTSD and associated sequelae in a community sample. Participants (female = 89%; black = 77%; aged 19–61) with PTSD were randomised to PTSD Coach-CS (n = 32) or enhanced Treatment-as-Usual (n = 30), and assessed with the Clinician-Administered PTSD Scale (CAPS-5), PTSD Checklist (PCL-5) and Depression, Anxiety and Stress Scale-21 items, at pre- to post-treatment and follow-up (1 and 3 months). We also collected data on user experiences of the PTSD Coach app with self-administered surveys. We conducted an intent-to-treat analysis and linear mixed models. A significant (group × time) effect for the CAPS-5 (F3.136 = 3.33, p = 0.02) indicated a greater reduction in PTSD symptom severity over time for the intervention group with a significant between-group effect size detected at 3-month follow-up. Significant between-group effect sizes were detected in self-reported stress symptom reduction in the intervention group at post-treatment and 3-month follow-up. Participants perceived the app as helpful and were satisfied with the app. Findings suggest PTSD Coach-CS as a suitable low-cost intervention and potential treatment alternative for adults with PTSD in a resource-constrained country. Replication in larger samples is needed to fully support effectiveness. Pan African Trial Registry: PACTR202108755066871.