Background: Food insecurity is a crucial but under-appreciated social determinant of health in GI cancers given high rates of cachexia where nutritional status may adversely impact functional status and outcomes. We investigated the impact of county-level food insecurity on age-adjusted mortality rates (AAMRs) from GI cancer. Methods: GI cancer-related deaths (ICD C15-26) were linked across US counties from 2015-2019 in the CDC WONDER database to county-level food insecurity measures from Feeding America and Census County Business Patterns databases. These included percent of individuals with food insecurity or on Supplemental Nutrition Assistance Program (SNAP) benefits, and the average cost/meal (USD) and density of local fast food/take-out restaurants. All measures were classified into quartiles based on distribution. AAMRs per 100,000 were compared between 1^st and 4^th quartiles for each measure using robust linear regression models with log scale and including the population size as weights. Results: There were 790,624 GI cancer deaths with overall AAMR 43.0 (68.5% age >65y, 57.8% male, 12.9% non-Hispanic Black, 8.9% Hispanic, 82.4% urban, 33.6% colorectal, 27.7% pancreatic, 19.6% liver/biliary cancer). Highest AAMRs were noted for age >65y, men, non-Hispanic White, and rural areas. AAMRs increased when moving from least to most insecure counties as defined by overall food insecurity, higher proportion of population on SNAP, or by higher local fast food density. AAMRs were lower in counties where cost per meal was higher. Association between AAMRs and all measures were strongest in younger adults (<65y), men, and rural counties, while association between AAMR and fast food density was strong among non-Hispanic Black individuals. Conclusions: Our study highlights the most food insecure US counties also have higher GI cancer mortality with significant sociodemographic variation. Food insecurity may be a helpful proxy for county-level social vulnerability in driving GI cancer mortality. Our findings recognize an important relationship between nutrition and cancer mortality and should inform ongoing congressional policy on food insecurity and assistance.