Objective Previous reviews have found equivocal evidence of an association between therapists’ internalized relational models and the therapeutic relationship and have neglected empirical literature based on Sullivan's notion of introject. This review expanded upon previous reviews to examine the effect of therapist internalized relational models on a broader conceptualization of the therapeutic relationship. Method Systematic search processes identified 22 papers measuring therapist attachment and/or introject and therapeutic relationship: 19 on therapist attachment, 5 on introject with 2 overlapping. Results Overall, despite heterogeneity in design and variable methodological quality, evidence suggests that therapist attachment affects therapeutic relationship quality, observed in client‐rated evaluation, therapist negative countertransference, empathy, and problems in therapy. Interaction effects between client and therapist attachment style were also found. Evidence suggesting that therapist introject also affects therapeutic relationship quality, including therapists’ manner and feelings toward their clients, was stronger. Conclusion Evidence clearly shows that therapists’ internalized relational models affect the therapeutic relationship. More research is necessary to clarify exactly how therapist and client internalized relational models interact and translate these findings into clinical practice.