Abstract High alcohol intake and breast density increase breast cancer (BC) risk, but their interrelationship is unknown. We examined whether volumetric density modifies and/or mediates the alcohol-BC association. BC cases (n = 2233) diagnosed from 2006–2013 in the San Francisco Bay Area had screening mammograms ≥6 months before diagnosis; controls (n = 4562) were matched on age, mammogram date, race/ethnicity, facility, and mammography machine. Logistic regression was used to estimate alcohol-BC associations, adjusted for age, body mass index, and menopause; interaction terms assessed modification. Percent mediation was quantified as the ratio of log(odds ratios [ORs]) from models with and without density measures. Alcohol consumption was associated with increased BC risk (two-sided p-trend = 0.004), as were volumetric percent density (OR = 1.45 per SD, 95%CI = 1.36–1.56) and dense volume (OR = 1.30, 95%CI = 1.24–1.37). Breast density did not modify the alcohol-BC association (two-sided p > .10 for all). Dense volume mediated 25.0% (95%CI = 5.5%–44.4%) of the alcohol-BC association (two-sided p = .01), suggesting alcohol may partially increase BC risk by increasing fibroglandular tissue.