IntroductionOver 80% of patients with Chiari Malformation (CM) struggle with moderate to severe disability due to neck pain and headaches. Self-guided online Acceptance and Commitment Therapy (ACT) is efficacious in addressing chronic pain. However, it is unclear whether adjunctive weekly phone coaching (a cost- and labor-intensive component of the intervention) contributes to, or provides additional benefit to, ACT's success. MethodThis study randomized 112 participants with CM to ACT + Coaching, ACT-Only, or WC (WC) groups. Participants completed a baseline survey, weekly assessments, a self-guided online ACT intervention, and a one-month follow-up survey. Primary outcomes were psychological flexibility (PF) and chronic pain acceptance; secondary outcomes included pain intensity, pain interference, depression, and anxiety. ResultsThere were no differences between the ACT + Coaching and ACT-Only groups on chronic pain acceptance, pain interference, or anxiety; however, there was a small but statistically significant effect favoring ACT + Coaching for depression and PF. Compared to the WC group, both the ACT + Coaching and ACT-Only groups significantly improved in chronic pain acceptance, and anxiety. Adherence was high (91% completed all sessions) and equivalent between conditions. Participants reported coaching was helpful and important, but there were no group differences in intervention satisfaction. DiscussionThere were no differences between treatment groups on most outcomes, intervention satisfaction, engagement, or adherence. Online ACT without adjunctive phone coaching may be a cost- and time-efficient option to address chronic pain acceptance and anxiety, but additional research is needed to establish and improve efficacy for other outcomes. Clinical Trials RegistrationNCT05581472