Background <p>Racial-ethnic differences in breast cancer treatment in the United States are well documented. Few studies have evaluated racial-ethnic differences in short-term outcomes following breast cancer surgery. This study compared postoperative outcomes following breast cancer surgery between racial-ethnic groups in the U.S. Military Health System.</p> Methods <p>We identified a cohort of women aged 18 and older diagnosed with stage I–III breast cancer between 2001 and 2014 who received partial (i.e., breast conserving, lumpectomy), total, or radical mastectomy without reconstruction in the MilCanEpi database. Multivariable Poisson regression estimated the adjusted risk ratios (ARRs) with 95% confidence intervals (CIs) in association with race-ethnicity for 30-day complications, reoperation, and hospital readmission.</p> Results <p>The study included 7527 women with mean age 56.2 (±12.8) years. The risk of 30-day complications was statistically significantly lower for Asian or Pacific Islander (<i>n</i> = 886; ARR = 0.67, 95% CI = 0.51, 0.89) and not statistically different for non-Hispanic Black (<i>n</i> = 1246; ARR = 0.81, 95% CI = 0.65, 1.02) and Hispanic (<i>n</i> = 497; ARR = 0.97, 95% CI = 0.73, 1.29) compared with Non-Hispanic White (<i>n</i> = 4,898) women. The risk of reoperation was not statistically different between the racial-ethnic groups. Hispanic women had a statistically significantly higher risk of 30-day readmission (ARR = 1.46, 95% CI = 1.08, 1.98). The risk was not statistically different for Asian and Black compared with White women.</p> Conclusions <p>In the Military Health System, the risk of 30-day complications and hospital readmissions varied between women of different race-ethnicity following surgery without reconstruction for nonmetastatic breast cancer. This suggests that factors other than availability and affordability of care in the differences and warrants further research.</p>