Louisiana is one of the few Southern states that enacted the Medicaid expansion of the Patient Protection and Affordable Care Act (ACA). To the authors' knowledge, the issue of how this has affected the breast cancer landscape in Louisiana is unknown. The authors have postulated that ACA expansion had a positive impact for Louisiana women diagnosed with breast cancer. Data from the Louisiana Tumor Registry regarding 14,640 women aged 20 to 64 years who resided in Louisiana and were diagnosed with American Joint Committee on Cancer stage 0 to stage IV breast cancer between 2012 and 2018 were analyzed. The study period was divided into 2 groups: 1) before ACA expansion (January 1, 2012-May 31, 2016); and 2) after ACA expansion (June 1, 2016-December 31, 2018). The chi-square test and multivariable logistic regression models were used to assess the impact of ACA expansion. A P value <.05 was considered statistically significant. After ACA expansion, the rate of uninsured patients decreased from 5.4% to 3.0% (P < .0001), and the rate of Medicaid recipients increased from 11.6% to 17.7% (P < .0001). The diagnosis of stage I breast cancer increased from 36.8% to 44.7% (P < .0001), whereas the diagnosis of stage III breast cancer decreased from 10.7% to 8.5% (P < .0001). The receipt of radiotherapy after breast-conserving surgery increased from 81.2% to 84.0% (P = .0035), and the receipt of radiotherapy within 90 days increased from 57.2% to 61.7% (P = .0012). After adjustment for sociodemographic and clinical variables, the models demonstrated that ACA expansion decreased the uninsured rate by 48% (odds ratio [OR], 0.52; 95% CI, 0.43-0.63), increased the diagnosis of early-stage disease (stage0 to stage II) by 27% (OR, 1.27; 95% CI, 1.15-1.41), increased receipt of radiotherapy after breast-conserving surgery by 19% (OR, 1.19; 95% CI, 1.03-1.37), and reduced the delay of receipt of radiotherapy by 16% (OR, 0.84; 95% CI, 0.74-0.95). ACA expansion in Louisiana reduced the uninsured rate, increased the diagnosis of early-stage disease, and increased access to treatment.