Breast cancer conservative treatment software (BCCT.core) has the potential to provide objective results using digital photographs, thus increasing the reliability, limiting the subjective interpretation and standardising the evaluation of breast cosmesis in patients. The aim of this systematic review was to summarise the empirical evidence regarding the agreement between the BCCT.core and other methods used by health professionals and patients to assess breast cosmesis. The Ovid MEDLINE, Embase, CINAHL, PsycINFO and Scopus databases were systematically searched for studies comparing the agreement between BCCT.core and other methods used to assess breast cosmesis. A total of 14 studies involving 2703 patients were included in the review. In studies where patients undertook self-assessment, the percentage agreement ranged from 69.2% to 74.8% and the kappa values ranged from slight (k = 0.12) to fair agreement. There was a low correlation between the BCCT.core and the Harvard scale when patients assessed cosmetic outcomes using photographs of their breasts (r = 0.165). A 75% and 42.8% agreement was reported in studies where clinicians assessed breast cosmesis by directly visualising the patients’ breasts and photographs, respectively. Assessment of breast cosmesis by the expert panel using patient photographs reported a percentage agreement ranging from 25% to 83%. The results of this systematic review demonstrated an overall agreement of fair to moderate between the BCCT.core and the subjective scales used to assess breast cosmesis. However, clinician experience and patient values should be considered in clinical decision-making.