Eating disorders are potentially life-threatening mental health disorders that require management by a multidisciplinary team including medical, psychological and dietetic specialties. This review systematically evaluated the available literature to determine the effect of including a dietitian in outpatient eating disorder (ED) treatment, and to contribute to the understanding of a dietitian's role in ED treatment. Six databases and Google Scholar were searched for articles that compared treatment outcomes for individuals receiving specialist dietetic treatment with outcomes for those receiving any comparative treatment. Studies needed to be controlled trials where outcomes were measured by a validated instrument (PROSPERO CRD42021224126). The searches returned 16,327 articles, of which 11 articles reporting on 10 studies were included. Two studies found that dietetic intervention significantly improved ED psychopathology, and three found that it did not. Three studies reported that dietetic input improved other psychopathological markers, and three reported that it did not. One consistent finding was that dietetic input improved body mass index/weight and nutritional intake, although only two and three studies reported on each outcome, respectively. A variety of instruments were used to measure each outcome type, making direct comparisons between studies difficult. Furthermore, there was no consistent definition of the dietetic components included, with many containing psychological components. Most studies included were also published over 20 years ago and are now out of date. Further research is needed to develop consistent dietetic guidelines and outcome measures; this would help to clearly define the role of each member of the multidisciplinary team, and particularly the role of dietitians, in ED treatment.