Background: Patients newly diagnosed with breast cancer face a series of complex decisions regarding locoregional and systemic treatment. There is a need to improve the quality of locoregional and systemic decisions for breast cancer patients, and to help patients understand the role of evaluative tests in this decision process. We are now conducting a randomized controlled trial (RCT) of an online decision tool—called iCanDecide, which we expect will help patients with these difficult decisions. Furthermore, the results of this RCT will be highly relevant to future breast cancer patients making these decisions and to their clinicians. Methods: This is a two-arm randomized controlled trial with the target of 222 participants per arm. Participants are recruited from 25 surgical practices (total 40 surgeons) and 2 medical oncology practices (total 2 oncologists) in Michigan, Georgia, Tennessee, and California. Participants are newly-diagnosed female breast cancer patients between 21 and 84 years, with stage I-II invasive breast cancer or ductal carcinoma in situ (DCIS) and who are eligible for and considering either mastectomy or lumpectomy with radiation, and who may be eligible for adjuvant systemic treatment. The RCT tests an interactive, tailored website, called iCanDecide (intervention arm), compared to a static version of the website (control arm). The static control arm is designed to include the same basic content as the intervention version, but without tailoring and interactive features. The primary outcome includes the rate of making a high-quality decision. The hypothesis is that patients randomized to the interactive version of iCanDecide will have higher rates of high quality decisions (informed and values-concordant), and will appraise their decision-making process more positively, for both surgical and systemic treatment. Discussion: The goal of this study is to evaluate the impact of the iCanDecide interactive website on decision-making for locoregional and systemic breast cancer treatments. The results of this study will be important for future breast cancer patients and their clinicians as we determine how to better individualize decision making across this complex treatment landscape. Trial registration: ClinicalTrials.gov ID NCT01840163.