Background:Once reserved for inoperable breast cancer, neoadjuvant systemic therapy (NAST) has become a treatment option for women with large operable or highly proliferative breast cancer. With equivalent survival outcomes between NAST and up-front surgery, the preference sensitive nature of the decision makes it suitable for a decision aid (DA). We aimed to develop and evaluate an online DA for women who had been offered NAST. Methods: Eligible women, considered by investigators as candidates for NAST, were enrolled in a single arm longitudinal study in Australia. Participants completed an online questionnaire prior to the DA, followed by a series of questionnaire post-DA up to 12 months from diagnosis. Primary outcomes were feasibility of use, and acceptability to patients and clinicians. Secondary outcomes were decision-related patient reported measures. Results: Seventy-nine women were screened, 59 were enrolled, and 47 (79.7%) reported having read the DA. 51 completed the first post-DA questionnaire. Patients were typically well educated, married, had health insurance and were information seekers. 41⁄51 (80.4%) of participants found the DA useful for their decision about NAST. 16⁄18 (88.9%) of responding investigators would continue to use the DA in routine practice. Post-DA, Decisional conflict decreased significantly across all subscales (p<0.01); anxiety and distress decreased significantly; 86.3% achieved at least as much decisional control as they desired; and a high level of knowledge was demonstrated. Discussion: This DA was feasible and acceptable to patients and clinicians, and improvements in decision-related outcomes were demonstrated. This DA could be implemented into routine practice for women with operable breast cancer who are candidates for NAST.