Existing measures of breast size dissatisfaction have poor ecological validity or have not been fully evaluated in terms of psychometric properties. Here, we report on the development of the Breast Size Rating Scale (BSRS), a novel measure of breast size dissatisfaction consisting of 14 computer-generated images varying in breast size alone. Study 1 (N = 107) supported the scale’s construct validity, insofar as participants were able to correctly order the images in terms of breast size. Study 2 (N = 234) provided evidence of the test-retest reliability of BSRS-derived scores after 3 months. Studies 3 (N = 730) and 4 (N = 234) provided evidence of the convergent validity of BSRS-derived breast size dissatisfaction scores, which were significantly associated with a range of measures of body image. The BSRS provides a useful tool for researchers examining women’s breast size dissatisfaction.