Background. The impact of the relationship (therapeutic alliance) between patients and physical therapists on treatment outcome in the rehabilitation of patients with chronic low back pain (LBP) has not been previously investigated. Objective. The purpose of this study was to investigate whether the therapeutic alliance between physical therapists and patients with chronic LBP predicts clinical outcomes. Design. This was a retrospective observational study nested within a randomized controlled trial. Methods. One hundred eighty-two patients with chronic LBP who volunteered for a randomized controlled trial that compared the efficacy of exercises and spinal manipulative therapy rated their alliance with physical therapists by completing the Working Alliance Inventory at the second treatment session. The primary outcomes of function, global perceived effect of treatment, pain, and disability were assessed before and after 8 weeks of treatment. Linear regression models were used to investigate whether the alliance was a predictor of outcome or moderated the effect of treatment. Results. The therapeutic alliance was consistently a predictor of outcome for all the measures of treatment outcome. The therapeutic alliance moderated the effect of treatment on global perceived effect for 2 of 3 treatment contrasts (general exercise versus motor control exercise, spinal manipulative therapy versus motor control exercise). There was no treatment effect modification when outcome was measured with function, pain, and disability measures. Limitations. Therapeutic alliance was measured at the second treatment session, which might have biased the interaction during initial stages of treatment. Data analysis was restricted to primary outcomes at 8 weeks. Conclusions. Positive therapeutic alliance ratings between physical therapists and patients are associated with improvements of outcomes in LBP. Future research should investigate the factors explaining this relationship and the impact of training interventions aimed at optimizing the alliance.