Abstract Background Limited and conflicting findings have been reported regarding the association between social support and colorectal cancer (CRC) outcomes. We sought to assess the influences of marital status and living arrangement on survival outcomes among patients with stage III colon cancer. Patients and Methods We conducted a secondary analysis of 1082 patients with stage III colon cancer prospectively followed in the CALGB 89803 randomized adjuvant chemotherapy trial. Marital status and living arrangement were both self-reported at the time of enrollment as, respectively, married, divorced, separated, widowed, or never-married, and living alone, with a spouse or partner, with other family, in a nursing home, or other. Results Over a median follow-up of 7.6 years, divorced/separated/widowed patients experienced worse outcomes relative to those married regarding disease free-survival (DFS) (hazards ratio (HR), 1.44 (95% CI, 1.14-1.81); P =.002), recurrence-free survival (RFS) (HR, 1.35 (95% CI, 1.05-1.73); P = .02), and overall survival (OS) (HR, 1.40 (95% CI, 1.08-1.82); P =.01); outcomes were not significantly different for never-married patients. Compared to patients living with a spouse/partner, those living with other family experienced a DFS of 1.47 (95% CI, 1.02-2.11; P = .04), RFS of 1.34 (95% CI, 0.91-1.98; P = .14), and OS of 1.50 (95% CI, 1.00-2.25; P =.05); patients living alone did not experience significantly different outcomes. Conclusion Among patients with stage III colon cancer who received uniform treatment and follow-up within a nationwide randomized clinical trial, being divorced/separated/widowed and living with other family were significantly associated with greater colon cancer mortality. Interventions enhancing social support services may be clinically relevant for this patient population. Trial Registration ClinicalTrials.gov Identifier: NCT00003835