Aim: To examine effects of diabetes mellitus (DM) on survival in gastric or esophageal (GE) cancer and the cancers’ effects on glycemic control. Materials & methods: Patients with GE cancer with and without DM were matched 1 to 1 (2006–2016). Characteristics were compared and survival assessed with Kaplan–Meier method and Cox regression. Mixed models compared hemoglobin A_1c and glucose over time. Results: Among DM cases, mean hemoglobin A_1c was 6.8% in the year after cancer diagnosis. Three-year overall survival was 46% with DM versus 52% without DM (hazard ratio [95% CI]: 1.95 [1.14–3.34]; p = 0.02). Conclusion: GE cancer and its treatment did not affect glycemic control. Risks of death and progression were greater for patients with DM than patients without DM.