Individuals with comorbid posttraumatic stress disorder (PTSD) and substance use disorder (SUD) present with a diversity of symptoms. Current interventions show minimal efficacy differences and have high attrition. Offering a variety of treatment options, including virtual ones, ensures treatment access that is appropriate and acceptable to individual needs. We conducted a single-arm pilot study to examine the acceptability and feasibility of an online intervention based on Internal Family Systems (IFS), called the Program for Alleviating and Reducing Trauma, Stress, and Substance Use (PARTS-SUD). Ten adults (N=10), with comorbid diagnoses of PTSD and SUD, were allocated to 12 weekly groups with 6 individual counseling sessions. Our pre-specified aims were acceptability (70% overall acceptability, 75% willingness to refer a friend), and feasibility (70% completion), with key exploratory clinical outcomes (PTSD symptom severity and craving). Participants rated the intervention with a mean score of 86% on acceptability, 92% on willingness to refer a friend, retaining 70% of participants at 12 weeks. Furthermore, PTSD symptoms reduced by 1.7 points/week (95% CI: -2.45, -0.93, p=0.002) with 54% of the sample achieving a minimally important difference in PCL-5 scores. Craving Scale scores were reduced by 0.25 points/week (95% CI: -0.45, -0.06, p=0.014). An online IFS intervention was a feasible and acceptable way to provide whole-person treatment for people with PTSD-SUD within a diverse community mental health center setting. Despite being a small pilot study, decreases in both PTSD symptom severity and craving indicate the need for a randomized controlled trial with a large, diverse sample.