A prospective, 2-armed, parallel group randomised controlled trial (RCT) was conducted to compare the effectiveness of Acceptance and Commitment Therapy (ACT) combined with a supervised exercise programme with a supervised exercise programme alone for adults with chronic pain. One hundred seventy-five participants were individually randomised to receive either the combined Exercise and ACT (ExACT) intervention or supervised exercise alone. Those allocated to the ExACT group attended 8 weekly sessions with a psychologist based on the ACT approach, in addition to supervised exercise classes led by a physiotherapist. The control group attended weekly supervised exercise classes but did not take part in an ACT programme. Both groups were followed up postintervention and again after 12 weeks. The primary outcome was pain interference at 12-week follow-up. Estimates of treatment effects at follow-up were based on intention-to-treat analyses, implemented using a linear mixed-effects model. The findings of this RCT showed no difference in the effectiveness of ExACT, compared with a supervised exercise programme alone for the primary outcome pain interference at 12-week follow-up (mean difference -0.18, 95% confidence interval -0.84 to 0.48, P = 0.59, d = 0.11). ExACT group participants reported superior outcomes for pain self-efficacy, pain catastrophising, and committed action, compared with the control group, but there were no differences between the groups for other secondary outcomes or treatment process measures. Higher levels of treatment satisfaction and global impression of change were reported by ExACT group participants. Exercise combined with Acceptance and Commitment Therapy was not superior to a standalone supervised exercise programme for reducing pain interference in adults with chronic pain.