Introduction Proper nutrition can play an important role in preventing and improving disease progression in patients with COVID-19. The Healthy Eating Index-2015 (HEI-2015) is one of the most common measures used to assess overall nutritional quality. This research aimed to identify the relationship between the HEI-2015 score and disease severity in hospitalised military patients with COVID-19. Methods This cross-sectional study was conducted in 295 hospitalised military patients (retired military and military reserve) with COVID-19. A validated food frequency questionnaire was used to assess food intake. To evaluate the quality of the diet, the HEI-2015 score was calculated. A multiple logistic regression analysis was performed to measure the association between HEI-2015 scores and disease severity (intensive care unit (ICU) admission and length of hospital stay greater than 4 days) in hospitalised military patients with COVID-19. Results The mean HEI-2015 score was significantly higher in non-ICU patients than in ICU patients (58.39±15.02 vs 53.54±15.65, p=0.01). After adjusting for possible confounding factors including age, sex, comorbidities, calorie intake, body mass index and physical activity, adherence to HEI-2015 inversely related to ICU admission (OR 0.98; 95% CI 0.95 to 1.00) and length of hospital stay of more than 4 days (OR 0.99; 95% CI 0.97 to 1.00) in hospitalised military patients with COVID-19, although statistically not significant. Conclusions According to the results of the study, adherence to HEI-2015 inversely related to both ICU admission and length of hospital stay in hospitalised military patients with COVID-19, although it was not statistically significant. WHAT IS ALREADY KNOWN ON THIS TOPIC ⇒ To our knowledge, this is the first study to examine the relationship between the Healthy Eating Index-2015 (HEI-2015) score and severity of illness in hospitalised military patients with COVID-19. WHAT THIS STUDY ADDS ⇒ The mean HEI-2015 score was significantly higher in non-intensive care unit (ICU) military patients than in ICU patients. ⇒ Adherence to HEI-2015 inversely related to ICU admission and length of hospital stay of more than 4 days in hospitalised military patients with COVID-19, although it was not statistically significant. HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY ⇒ It appears that a diet rich in fresh fruits and vegetables, whole grains, protein foods, dairy products and healthy fats could be effective in reducing the severity of COVID-19 disease.