PURPOSE: Suboptimal diet is a preventable cause of cancer. We aimed to estimate the economic burden of diet-associated cancer among US adults. METHODS: We used a Comparative Risk Assessment model to quantify the number of new cancer cases attributable to seven dietary factors among US adults ages 20 + years. A Markov cohort model estimated the 5-year medical costs for 15 diet-associated cancers diagnosed in 2015. We obtained dietary intake from 2013 to 2016 National Health and Nutrition Examination Survey, cancer incidence, and survival from 2008 to 2014 Surveillance, Epidemiology, and End Results (SEER) program, and medical costs from 2007 to 2013 linked SEER-Medicare data. RESULTS: The estimated 5-year medical costs of new diet-associated cancer cases diagnosed in 2015 were $7.44 (2018 US$). Colorectal cancer had the largest diet-related 5-year medical costs of $5.32B. Suboptimal consumption of whole grains ($2.76B), dairy ($1.82B), and high consumption of processed meats ($1.5B) accounted for the highest medical costs. Per-person medical costs attributable to suboptimal diet vary by gender, race, and age group. CONCLUSIONS: Suboptimal diet contributes substantially to the economic burden of diet-associated cancers among US adults. This study highlights the need to implement population-based strategies to improve diet and reduce cancer burden in the US.