Background: Prior studies have supported an inverse association between physical activity and colon cancer risk and suggest that higher physical activity may also improve cancer survival. Among participants in a phase III adjuvant trial for stage III colon cancer, we assessed the association of physical activity around the time of cancer diagnosis with subsequent outcomes.Methods: Before treatment arm randomization (FOLFOX or FOLFOX + cetuximab), study participants completed a questionnaire including items regarding usual daily activity level and frequency of participation in recreational physical activity (N = 1,992). Using multivariable Cox models, we calculated HRs for associations of aspects of physical activity with disease-free (DFS) and overall survival (OS).Results: Over follow-up, 505 participants died and 541 experienced a recurrence. Overall, 75% of participants reported recreational physical activity at least several times a month; for participants who reported physical activity at least that often (vs. once a month or less), the HRs for DFS and OS were 0.82 [95% confidence interval (CI), 0.69-0.99] and 0.76 (95% CI, 0.63-0.93), respectively. There was no evidence of material effect modification in these associations by patient or tumor attributes, except that physical activity was more strongly inversely associated with OS in patients with stage T3 versus T4 tumors (Pinteraction = 0.03).Conclusions: These findings suggest that higher physical activity around the time of colon cancer diagnosis may be associated with more favorable colon cancer outcomes. Impact: Our findings support further research on whether colon cancer survival may be enhanced by physical activity.