Objective This study aims at evaluating the cost-effectiveness and cost-utility of a guided and unguided internet-based intervention for chronic pain patients (ACTonPain guided and ACTonPain unguided) compared with a waitlist control group (CG) as well as the comparative cost-effectiveness of the guided and the unguided version. Design This is a health economic evaluation alongside a three-arm randomised controlled trial from a societal perspective. Assessments were conducted at baseline, 9 weeks and 6 months after randomisation. Setting Participants were recruited through online and offline strategies and in collaboration with a health insurance company. Participants 302 adults (≥18 years, pain for at least 6 months) were randomly allocated to one of the three groups (ACTonPain guided, ACTonPain unguided, CG). Interventions ACTonPain consists of seven modules and is based on Acceptance and Commitment Therapy. ACTonPain guided and ACTonPain unguided only differ in provision of human support. Primary and secondary outcome measures Main outcomes of the cost-effectiveness and the cost-utility analyses were meaningful change in pain interference (treatment response) and quality-adjusted life years (QALYs), respectively. Economic evaluation estimates were the incremental cost-effectiveness and cost-utility ratio (ICER/ICUR). Results At 6-month follow-up, treatment response and QALYs were highest in ACTonPain guided (44% and 0.280; mean costs = €6,945), followed by ACTonPain unguided (28% and 0.266; mean costs = €6,560) and the CG (16% and 0.244; mean costs = €6,908). ACTonPain guided vs CG revealed an ICER of €45 and an ICUR of €604.ACTonPain unguided dominated CG. At a willingness-to-pay of €0 the probability of being cost-effective was 50% for ACTonPain guided (vs CG, for both treatment response and QALY gained) and 67% for ACTonPain unguided (vs CG, for both treatment response and QALY gained). These probabilities rose to 95% when society's willingness-to-pay is €91,000 (ACTonPain guided) and €127,000 (ACTonPain unguided) per QALY gained. ACTonPain guided vs ACTonPain unguided revealed an ICER of €2,374 and an ICUR of €45,993. Conclusions Depending on society's willingness-to-pay, ACTonPain is a potentially cost-effective adjunct to established pain treatment. ACTonPain unguided (vs CG) revealed lower costs at better health outcomes. However, uncertainty has to be considered. Direct comparison of the two interventions does not indicate a preference for ACTonPain guided. Trial registration number DRKS00006183.