Posttraumatic stress disorder (PTSD) and alcohol use disorder (AUD) frequently co‐occur and are associated with worse outcomes together than either disorder alone. A lack of consensus regarding recommendations for treating PTSD–AUD exists, and treatment dropout is a persistent problem. Acceptance and Commitment Therapy (ACT), a transdiagnostic, mindfulness‐ and acceptance‐based form of behavior therapy, has potential as a treatment option for PTSD–AUD. In this uncontrolled pilot study, we examined ACT for PTSD–AUD in 43 veterans; 29 (67%) completed the outpatient individual therapy protocol (i.e., ≥ 10 of 12 sessions). Clinician‐assessed and self‐reported PTSD symptoms were reduced at posttreatment, ds = 0.79 and 0.96, respectively. Self‐reported symptoms of PTSD remained lower at 3‐month follow‐up, d = 0.88. There were reductions on all alcohol‐related outcomes (clinician‐assessed and self‐reported symptoms, total drinks, and heavy drinking days) at posttreatment and 3‐month follow‐up, dmean = 0.91 (d range: 0.65–1.30). Quality of life increased at posttreatment and follow‐up, ds = 0.55–0.56. Functional disability improved marginally at posttreatment, d = 0.35; this effect became significant by follow‐up, d = 0.52. Fewer depressive symptoms were reported at posttreatment, d = 0.50, and follow‐up, d = 0.44. Individuals experiencing suicidal ideation reported significant reductions by follow‐up. Consistent with the ACT theoretical model, these improvements were associated with more between‐session mindfulness practice and reductions in experiential avoidance and psychological inflexibility. Recommendations for adapting ACT to address PTSD–AUD include assigning frequent between‐session mindfulness practice and initiating values clarification work and values‐based behavior assignments early in treatment.