© 2015, Springer International Publishing Switzerland. Purpose: Data relating to magnesium intake and colorectal cancer (CRC) risk in postmenopausal women are incomplete. We investigated the association between total magnesium intake and the risk of CRC in an ethnically diverse cohort of postmenopausal women enrolled in the Women’s Health Initiative. Methods: Self-reported dietary and supplemental magnesium were combined to form total magnesium intake. Invasive incident CRC was the primary outcome. Cox proportional hazard models were used to estimate hazard ratios (HRs) and 95 % confidence intervals (CI). Results: During an average follow-up of 13 years (1,832,319 person-years), of the 140,601 women included for analysis, 2,381 women were diagnosed with CRC (1,982 colon cancer and 438 rectal cancer). After adjustment for potential confounding variables, an inverse association was observed in the highest quintile of total magnesium intake compared to the lowest quintile for risk of CRC (HR 0.79, 95 % CI 0.67, 0.94, ptrend < 0.0001) and colon cancer (HR 0.80, 95 % CI 0.66, 0.97, ptrend < 0.0001). A borderline significant inverse association was detected in the highest versus the lowest quintile of total magnesium intake for rectal cancer (HR 0.76, 95 % CI 0.51, 1.13, ptrend < 0.001). Conclusions: Findings from this study support the hypothesis that magnesium intake around 400 mg/day from both dietary and supplemental sources is associated with a lower incidence of CRC in postmenopausal women.