Objective <p>To analyze the risk factors for postoperative secondary upper limb lymphedema in patients with locally advanced triple-negative breast cancer (TNBC).</p> Methods <p>In this retrospective cohort study, a total of 176 patients with stage IIIB–IIIC TNBC treated at our hospital from January 2021 to December 2023 were included consecutively. The modified technique was introduced in 2022, and all procedures were performed by the same surgical team. They were divided into an experimental group (modified surgical technique, <i>n</i> = 90) and a control group (standard technique, <i>n</i> = 86) based on the surgical approach. The modified technique involved concurrent resection of the clavicular portion of the pectoralis major muscle fibers and the clavipectoral fascia in addition to standard axillary lymph node dissection (ALND). The incidence of postoperative upper limb lymphedema was compared between groups, and its association with obesity, cardiovascular disease, and metabolic syndrome was analyzed.</p> Results <p>The postoperative lymphedema incidence was 23.3% (21/90) in the experimental group, significantly lower than the 41.9% (36/86) in the control group (<i>P</i> = 0.009). Lymphedema incidence in obese patients was 47.8% (43/90), markedly higher than the 16.3% (14/86) in non-obese patients (<i>P</i> &lt; 0.001). Multivariate logistic regression identified obesity (OR = 4.80, 95% CI 2.38–9.68, <i>P</i> &lt; 0.001) and standard surgical technique (OR = 2.45, 95% CI 1.23–4.88, <i>P</i> = 0.011) as independent risk factors for lymphedema.</p> Conclusion <p>Obesity and surgical technique are independent risk factors for postoperative upper limb lymphedema in locally advanced TNBC patients. The modified ALND technique may help reduce lymphedema incidence, with a particularly pronounced benefit observed in obese patients. However, the proposed mechanism remains speculative, and the findings require prospective validation. Prospective randomized studies are needed before routine adoption.</p>