There is a need for a new, less invasive breast reconstruction option for patients who undergo mastectomy in their breast cancer treatment. To investigate quality of life (QoL) among patients undergoing a new breast reconstruction technique, autologous fat transfer (AFT), compared with that among patients undergoing implant-based reconstruction (IBR). The BREAST trial was a randomized clinical trial conducted between November 2, 2015, and October 31, 2021, performed in 7 hospitals across the Netherlands. Follow-up was 12 months. Referrals could be obtained from general practitioners and all departments from participating or nonparticipating hospitals. The patients with breast cancer who had undergone mastectomy and were seeking breast reconstruction were screened for eligibility (radiotherapy history and physique) by participating plastic surgeons. Patients receiving postmastectomy radiotherapy were excluded. Breast reconstruction with AFT plus expansion or 2-phased IBR. Randomization was done in a 1:1 ratio. The statistical analysis was performed per protocol. The predefined primary outcome was QoL at 12 months after final surgery. This was measured by the BREAST-Q questionnaire, a validated breast reconstruction surgery questionnaire. Questions on the BREAST-Q questionnaire are scored from 0 to 100, with a higher score indicating greater satisfaction or better QoL (depending on the scale). Secondary outcomes were breast volume and the safety and efficacy of the techniques. A total of 193 female patients (mean [SD] age, 49.2 [10.6] years) 18 years or older who desired breast reconstruction were included, of whom 91 patients in the AFT group (mean [SD] age, 49.3 [10.3] years) and 80 in the IBR group (mean age, 49.1 [11.0] years) received the allocated intervention. In total, 64 women in the AFT group and 68 women in the IBR group completed follow-up. In the IBR group, 18 patients dropped out mainly due to their aversion to implant use while in the AFT group 6 patients ended their treatment prematurely because of the burden (that is, the treatment being too heavy or tiring). The BREAST-Q scores were higher in the AFT group in all 5 domains and significantly higher in 3: satisfaction with breasts (difference, 9.9; P = .002), physical well-being: chest (difference; 7.6; P = .007), and satisfaction with outcome (difference, 7.6; P = .04). Linear mixed-effects regression analysis showed that QoL change over time was dependent on the treatment group in favor of AFT. The mean (SD) breast volume achieved differed between the groups (AFT: 300.3 [111.4] mL; IBR: 384.1 [86.6] mL). No differences in oncological serious adverse events were found. This randomized clinical trial found higher QoL and an increase in QoL scores over time in the AFT group compared with the IBR group. No evidence was found that AFT was unsafe. This is encouraging news since it provides a third, less invasive reconstruction option for patients with breast cancer. ClinicalTrials.gov Identifier: NCT02339779.