Treating patients with eating disorders can be challenging for therapists, as it requires the establishment of a strong therapeutic relationship. According to the literature, therapist characteristics may influence intervention outcomes. The aim of this systematic review was to identify and synthesize existing literature on therapist interpersonal characteristics that could affect psychotherapy relationship or outcomes in the context of eating disorder treatment from both patients' and therapists' perspectives. We conducted a systematic search using electronic databases and included both qualitative and quantitative studies from 1980 until July 2023. Out of the 1230 studies screened, 38 papers met the inclusion criteria and were included in the systematic review. The results indicate that patients reported therapist's warmth, empathic understanding, a supportive attitude, expertise in eating disorders, and self-disclosure as positive characteristics. Conversely, a lack of empathy, a judgmental attitude, and insufficient expertise were reported as therapist negative characteristics, which could have a detrimental impact on treatment outcome. Few studies have reported therapist's perceptions of their own personal characteristics which could have an impact on treatment. Therapists reported that empathy and supportiveness, optimism, and previous eating disorder experience were positive characteristics. Conversely, clinician anxiety, a judgmental attitude, and a lack of objectivity were reported as negative characteristics that therapists felt could hinder treatment. This systematic review offers initial evidence on the personal characteristics of therapists that may affect the treatment process and outcomes when working with patients with eating disorders.