Differences between therapists in the average outcomes their patients achieve are well documented, and researchers have begun to try to explain such differences (Baldwin & Imel, 2013). Guided by Self-Determination Theory (Deci & Ryan, 2000), we examined the effects on outcome of differences between therapists in their patients’ average levels of autonomous and controlled motivation for treatment, as well as the effects of differences among the patients within each therapist’s caseload. Between and within-therapist differences in the SDT construct of perceived relational support were explored as predictors of patients’ motivation. Nineteen therapists treated 63 patients in an outpatient clinic providing manualized interpersonal therapy (IPT) for depression. Patients completed the BDI-II at pretreatment, posttreatment, and each treatment session. The Impact Message Inventory was administered at the third session and scored for perceived therapist friendliness, a core element of relational support. We created between-therapists (therapist-level) scores by averaging over the patients in each therapist’s caseload; within-therapist (patient-level) scores were computed by centering within each therapist’s caseload. As expected, better outcome was predicted by higher levels of therapist-level and patient-level autonomous motivation and by lower levels of therapist-level and patient-level controlled motivation. In turn, autonomous motivation was predicted by therapist-level and patient-level relational support (friendliness). Controlled motivation was predicted solely by patient self-critical perfectionism. The results extend past work by demonstrating that both between-therapists and within-therapist differences in motivation predict outcome. As well, the results suggest that therapists should monitor their interpersonal impact so as to provide relational support.