Abstract Female breast cancer is a common cancer in young adults, an age group with the highest uninsured rate. Among 51 675 young adult women (ages 18-39 years) diagnosed with breast cancer between 2011 and 2018 in the National Cancer Database, we estimated changes in guideline-concordant treatment receipt, treatment timeliness, and survival associated with the Affordable Care Act Medicaid expansion. Of young adults with stage I-III estrogen receptor–positive or progesterone receptor–positive breast cancer, Medicaid expansion was associated with a net increase of 2.42 percentage points (95% confidence interval [CI] = 0.56 to 4.28 percentage points) in the percentage receiving endocrine therapy. Among all young adults with stage I-III breast cancer, Medicaid expansion was associated with a net reduction of 1.65 percentage points (95% CI = 0.08 to 3.22 percentage points) in treatment delays defined as treatment initiation of at least 60 days after diagnosis and a net increase of 1.00 percentage points (95% CI = 0.21 to 1.79 percentage points) in 2-year overall survival. Our study provides evidence of benefit in cancer care and outcomes from Medicaid expansion among the young adult population.