Background: HER2-enriched breast cancer (estrogen and progesterone receptors negative and HER2-positive) has no worse prognosis than other breast cancers if it is treated with HER2-targeted therapy. Medicaid expansion under the Affordable Care Act (ACA) has been shown to be associated with improved access to care and outcomes for many cancers, but its impact on care for HER2-enriched breast cancer is unknown. To fill this gap, we examined the association of Medicaid expansion with receipt of guideline-concordant treatment, time to treatment, and survival among newly diagnosed nonelderly women with HER2-enriched breast cancer. Methods: Women aged 18-62 years newly diagnosed with HER2-enriched breast cancer between 2010 and 2018 were identified from the National Cancer Database. Our primary outcomes included receipt of stage-based guideline-concordant treatment, initiation of treatment <60 days, and stage-specific 2-year overall survival. A difference-in-differences (DD) approach compared outcome changes following the expansion in Medicaid expansion vs. non-expansion states. A linear probability model estimated treatment outcome, and a flexible parametric survival model evaluated survival. All models were clustered at the state level and adjusted for age group, race/ethnicity, rurality of residence, zip code-level income, comorbidity, and diagnosis year. Results: A total of 31,354 patients with HER2-enriched breast cancer were included, including 19,255 in expansion states and 12,099 in non-expansion states. Compared to patients in expansion states, patients in non-expansion states were more likely to be racial/ethnic minorities, uninsured, and diagnosed at late-stage. Medicaid expansion was not significantly associated with changes in receipt of surgery for stage I-III disease but was associated with an increase of 4.42 percentage points (ppt, 95% CI: 0.09, 8.74) in receipt of chemotherapy/targeted therapy for stage IV disease, with an increase from 90.4% pre-expansion to 92.8% post-expansion in expansion states and a decrease from 93.7% to 91.9% in non-expansion states. Medicaid expansion was also significantly associated with a 2.41 ppt increase in initiating guideline-concordant treatment within 60 days (95% CI: 0.66, 4.17; 83.5% to 82.5% in expansion states vs. 86.6% to 83.7% in non-expansion states), and a 1.18 ppt increase in two-year survival rate (95% CI: 0.03, 2.33; 94.0% to 94.9% in expansion states vs. 94.3% to 94.1% in non-expansion states). The increase in two-year survival associated with Medicaid expansion was most prominent for stage III patients (DD=3.63 ppt; 95% CI: 0.64, 6.62). Conclusions: Medicaid expansion was associated with increased receipt of timely treatment and improved 2-year survival among nonelderly women newly diagnosed with HER2-enriched breast cancer.