Objective: Previous studies have suggested that transportation noise may increase risk for breast cancer, but existing literature is scarce and inconclusive. We aimed to investigate associations between road traffic and railway noise and risk for breast cancer across the entire Danish female population. Methods: For all 2.8 million residential addresses across Denmark, we modelled road and railway noise at the most and least exposed façades for the period 1990–2017. We calculated 10-year time-weighted mean noise exposure for 1.8 million women aged >35 years, of whom 66,006 developed breast cancer during follow-up from 2000 to 2017. We analysed data using Cox proportional hazards models with noise exposure included as 10-year running means and adjusted for a number of individual and area-level socioeconomic co-variates and air pollution with fine particles estimated for all addresses. Results: For exposures at the least exposed façade, we found that a 10 dB increase in 10-year time-weighted noise was associated with incidence rate ratios (IRRs) and 95% confidence intervals (CI) for breast cancer of 1.032 (1.019–1.046) for road noise and 1.023 (0.993–1.053) for railway noise. For exposures at the most exposed façade, the IRRs (95% CIs) were 1.012 (1.002–1.022) for road noise and 1.020 (1.001–1.039) for railway noise. Associations were strongest among women with human epidermal growth factor receptor 2 negative breast cancer. Conclusions: Road traffic and railway noise were associated with higher risk for breast cancer, especially noise at the least exposed façade, which is a proxy for noise exposure during sleep.