Objective: There are few effective treatments for acute whiplash-associated disorders (WAD). Early symptoms of postinjury stress predict poor recovery. This randomised controlled trial (StressModex) investigated whether physiotherapist-led stress inoculation training integrated with exercise is more effective than exercise alone for people with acute WAD. Methods: 108 participants (<4 weeks) at risk of poor recovery (moderate pain-related disability and hyperarousal symptoms) were randomly assigned by concealed allocation to either physiotherapist-led stress inoculation training and guideline-based exercise (n=53) or guideline-based exercise alone (n=55). Both interventions comprised 10 sessions over 6 weeks. Participants were assessed at 6 weeks and at 6 and 12 months postrandomisation. Analysis was by intention to treat using linear mixed models. Results: The combined stress inoculation training and exercise intervention was more effective than exercise alone for the primary outcome of pain-related disability at all follow-up points. At 6 weeks, the treatment effect on the 0–100 Neck Disability Index was (mean difference) −10 (95% CI −15.5 to −4.48), at 6 months was −7.8 (95% CI −13.8 to −1.8) and at 12 months was −10.1 (95% CI −16.3 to −4.0). A significant benefit of the stress inoculation and exercise intervention over exercise alone was also found for some secondary outcomes. Conclusion: A physiotherapist-led intervention of stress inoculation training and exercise resulted in clinically relevant improvements in disability compared with exercise alone—the most commonly recommended treatment for acute WAD. This contributes to the case for physiotherapists to deliver an early psychological intervention to patients with acute WAD who are otherwise at high risk of a poor outcome.