Background: This study evaluated the impact of diabetes mellitus (DM) on survival in gastric and esophageal cancer patients, and examined the impact of these cancers on glycemic control in DM. Methods: Patients with newly diagnosed gastric and esophageal cancers with DM (n = 92) were identified from the Institutional Cancer Registry and matched to 92 gastric and esophageal cancer patients without DM according to age, gender and year of cancer diagnosis (2006 to 2016). The electronic medical record provided information on DM and cancer therapies and laboratory results. Overall survival (OS) was estimated with the Kaplan-Meier method and compared by Cox regression analysis using stratification for matched pairs. Mixed models were used to compare hemoglobin A1c (HbA1c) and glucose during the year following cancer diagnosis. Results: Mean age of the entire cohort was 68 years, 91% were white, 78% were men, and 53% had stage III/IV disease. Adenocarcinoma (79%) was the most common histologic type. BMI was significantly different between DM and non-DM patients (p = 0.006). Alcohol use at time of cancer diagnosis was more prevalent in non-DM patients vs. DM (p = 0.018). Among those with DM, mean HbA1c during the year following cancer diagnosis was 6.8%. Mean glucose was significantly different between DM and non-DM patients (149 mg/dL vs. 116 mg/dL, p < 0.001). For glucose, there was a significant interaction effect (p = 0.005) as DM patients demonstrated a decrease in glucose values over time compared to non-DM patients. Median follow-up time was 35 months. Three year OS was estimated at 46% (95% CI: 36-58%) for DM patients versus 52.0% (95% CI: 41-64%) in non-DM (p = 0.25). Hazard ratio (stratification for matched pairs) was 1.95 (95% CI: 1.14 – 3.43; p = 0.02). Three year PFS was estimated at 40% (95% CI: 31-53%) for DM patients versus 50% (95% CI: 40-63%) for non-DM patients (p = 0.12). Hazard ratio (stratification for matched pairs) was 1.74 (95% CI: 1.04-2.90; p = 0.03). Conclusions: Gastric and esophageal cancer and its treatment did not affect glycemic control. Risks of death and progression are greater in DM patients as compared to non-DM.