Patients with early breast cancer must choose between undergoing breast conservation surgery or mastectomy. This decision is often difficult as there are trade-offs between breast conservation and adverse effects, and women with higher decisional conflict have a harder time choosing the therapy that suits their preferences. To study the impact of a decision aid with a patient preference assessment tool for surgical decision-making on patients' decisional conflict scale (DCS) score. This 3-group randomized clinical trial was conducted between June 2017 and December 2019 at a single high-volume tertiary care cancer center in Mumbai, India. A research questionnaire comprising 16 questions answered on a Likert scale (from 1, strongly agree, to 5, strongly disagree) was used to measure DCS scores and other secondary psychological variables, with higher scores indicating more decisional conflict. The Navya Patient Preference Tool (Navya-PPT) was developed as a survey-based presentation of evidence in an adaptive, conjoint analysis-based module for and trade-offs between cosmesis, adverse effects of radiotherapy, and cost of mandatory radiation following breast-conserving surgery. Adult patients with histologically proven early breast cancer (cT1-2, N0-1) who were eligible for breast-conserving surgery as per clinicoradiological assessment were included. Those who were pregnant or unable to read the research questionnaire or who had bilateral breast cancer were excluded. Data were analyzed from January to June 2020. Patients were randomized 1:1:1 to study groups: standard care including clinical explanation about surgery (control), standard care plus the Navya-PPT provided to the patient alone (solo group), and standard care plus the Navya-PPT provided to the patient and a caregiver (joint group). The primary end point of the study was DCS score. The study was 80% powered with 2-sided α = .01 to detect an effect size of 0.25 measured by Cohen d, F test analysis of variance, and fixed effects. A total of 245 female patients (median [range] age, 48 [23-76] years) were randomized (82 to control, 83 to the solo group, and 80 to the joint group). The median (range) pathological tumor size was 2.5 (0-6) cm. A total of 153 participants (62.4%) had pN0 disease, 185 (75.5%) were hormone receptor positive, 197 (80.4%) were human epidermal growth factor receptor 2 negative, 144 (58.6%) were of middle or lower socioeconomic status, and 114 (46.5%) had an education level lower than a college degree. DCS score was significantly reduced in the solo group compared with control (1.34 vs 1.66, respectively; Cohen d, 0.50; SD, 0.31; P < .001) and the joint group compared with control (1.31 vs 1.66, respectively; Cohen d, 0.54; SD, 0.31; P < .001). The results of this study demonstrated lower decisional conflict as measured by DCS score following use of the online, self-administered Navya-PPT among patients with early breast cancer choosing between breast-conserving surgery vs mastectomy. Clinical Trials Registry of India Identifier: CTRI/2017/11/010480.