Grounded in a view of the therapeutic alliance as a process of intersubjective negotiation between patient and therapist, this study examines therapist reflective functioning (RF) as a predictor of process and outcome of psychotherapy in 43 cases of brief relational therapy. Psychotherapy process was measured with the Working Alliance Inventory, Session Evaluation Questionnaire, and a measure of rupture resolution. Outcome was measured with the Symptom Checklist Revised—90 (SCL-90) and the Inventory of Interpersonal Problems—32 (IIP-32) at intake, termination, and 6-month follow-up. Analyses revealed that higher therapist RF predicted greater therapist-reported Working Alliance Inventory, greater patient-reported depth, and greater reported degree of resolving ruptures from both patients’ and therapists’ perspectives. Therapist RF was correlated with increased self-reported symptoms on SCL-90 and IIP-32 from intake to termination. Therapist RF was correlated with a decrease in symptoms on SCL-90 and interpersonal problems on IIP-32 from termination to 6-month follow-up. The results are consistent with the hypothesis that therapists’ capacity for mentalization is associated with greater depth of in-session exploration and greater success in resolving in-session ruptures allowing for the potential facilitation of greater patient change. (PsycINFO Database Record (c) 2019 APA, all rights reserved)