Therapists' in-session feelings in psychotherapy can be seen as indications of the development of the therapeutic relationship and the therapeutic process. To manage them appropriately, it is important to know to what extent they may be influenced by patients' pretreatment characteristics. This study aims to improve the understanding of therapists' emotional reactions in the psychotherapeutic setting by investigating if patients' pretreatment mentalization ability and attachment style predicted therapist in-session feelings. In a sample of 87 therapy dyads treated with interpersonal psychotherapy and cognitive behavioral therapy for depression, patient attachment was measured using self-reported Experiences in Close Relationships (ECR) and mentalization using Reflective Functioning (RF). ECR and RF were hypothesized to predict therapist feelings measured by the Feeling Word Checklist-24 at different treatment phases over the full course of treatment. Treatment method, patient age, gender, and pretreatment depression were evaluated as potential confounders. Multilevel modeling was used to analyze the data. Lower RF in patients predicted more negative therapist feelings in the mid- to late-treatment phases and less positive feelings in the late-treatment phase. Self-reported attachment anxiety or avoidance did not predict therapist feelings. Findings indicate that patients' ability to mentalize is important to consider when conducting psychotherapy, as it can influence therapists' feelings in the therapeutic process. Limitations of the present study's approach are discussed, and directions for future research are considered.