The goal of this study was to examine the association of breast arterial calcification (BAC) presence and quantity with incident atrial fibrillation (AF) in a large cohort of post-menopausal women. We conducted a longitudinal cohort study among women free of clinically overt cardiovascular disease and AF at baseline (between October 2012 and February 2015) when they attended mammography screening. Atrial fibrillation incidence was ascertained using diagnostic codes and natural language processing. Among 4908 women, 354 incident cases of AF (7%) were ascertained after a mean (standard deviation) of 7 (2) years of follow-up. In Cox regression adjusting for a propensity score for BAC, BAC presence vs. absence was not significantly associated with AF [hazard ratio (HR) = 1.12; 95% confidence interval (CI), 0.89-1.42; P = 0.34]. However, a significant (a priori hypothesized) age by BAC interaction was found (P = 0.02) such that BAC presence was not associated with incident AF in women aged 60-69 years (HR = 0.83; 95% CI, 0.63-1.15; P = 0.26) but was significantly associated with incident AF in women aged 70-79 years (HR = 1.75; 95% CI, 1.21-2.53; P = 0.003). No evidence of dose-response relationship between BAC gradation and AF was noted in the entire cohort or in age groups separately. Our results demonstrate, for the first time, an independent association between BAC and AF in women over age 70 years.