Purpose To assess the prognostic value of pneumonia severity score (PSS), pectoralis muscle area (PMA), and index (PMI) on chest computed tomography (CT) in adult coronavirus disease 2019 (COVID-19) patients. Method The chest CT images of COVID-19 patients were evaluated for the PSS as the ratio of the volume of involved lung parenchyma to the total lung volume. The cross-sectional areas of the pectoralis muscles (PMA, cm2) were also measured automatically on axial CT images, and PMI was calculated as the following formula: PMI = PMA / patient’s height square (m2). The relationship between clinical variables, PSS, PMA, sex-specific PMI values, and patient outcomes (intubation, prolonged hospital stay, and death) were investigated using multivariable logistic regression analysis. All patients were followed for more than a month. Results One-hundred thirty patients (76 males, 58.46 %) were included in the study. Fifteen patients (11.54 %) were intubated, 24 patients (18.46 %) had prolonged hospital stay, and eight patients (6.15 %) died during follow-up. Patients with comorbidity had a higher mean of PSS (6.3 + 4.5 vs 3.9 + 3.8; p = 0.001). After adjusting the confounders, PSS was an independent predictor of intubation (adjusted Odds Ratio [OR]: 1.73, 95 % CI 1.31−2.28, p < 0.001), prolonged hospital stay (OR: 1.20, 95 % CI 1.09−1.33, p < 0.001), and death (OR: 2.13, 95 % CI 1.1−4.13, p = 0.026. PMI value was a predictor of prolonged hospital stay (OR: 0.83, 95 % CI 0.72−0.96, p = 0.038) and death (OR: 0.53, 95 % CI 0.29−0.96, p = 0.036). Incrementally increasing PMA value was a predictor of prolonged hospital stay (OR: 0.93, 95 % CI 0.89−0.98, p = 0.01) and intubation (OR: 0.98, 95 % CI 0.96−1, p = 0.036). Conclusion PSS, PMA, and PMI values have prognostic value in adult COVID-19 patients and can be easily assessed on chest CT images.