Objective: To assess the effectiveness of high-dose corticosteroid pulse therapy and evaluate possible factors associated with 28-day mortality in hospitalised patients with severe COVID-19 pneumonia. Methods: We conducted a single-centre retrospective cohort study on hospitalised patients with clinical, epidemiological, and/or radiologically confirmed and suspected COVID-19 at Bitlis Tatvan State Hospital in Turkey between December 1, 2020 and June 1, 2021. All data of the study participants were recorded, and all patients received intravenous high-dose corticosteroid pulse therapy. The Ordinal Scale for Clinical Improvement (OSCI), Charlson Comorbidity Index and Total Severity Score were calculated. Univariate and multivariate Cox regression models were performed to evaluate the clinical and laboratory parameters that may affect the 28-day mortality. Results: A total of 126 patients were included in the analysis. The 28-day mortality rate of the patients was 22.2%. Laboratory and clinical improvement were observed in 77.8% (98/126) of patients after high-dose corticosteroid pulse therapy. There was a statistically significant difference between the survivors and non-survivors in terms of age, platelet count, neutrophil/lymphocyte ratio, and OSCI, Charlson Comorbidity Index, and Total Severity Score (P<0.001). Multivariate Cox regression analysis revealed that age [HR 1.047 (95% CI 1.01-1.08)], use of prophylactic anticoagulation [HR 0.838 (95% CI 0.79-0.89)], and bacterial co-infection [HR 3.966 (95% CI 1.40-11.21)] were significant determinants of mortality. Early C-reactive protein (CRP) response, decreased oxygen requirement, and improving respiratory rate/OSCI scores after administration of high-dose corticosteroid pulse therapy could contribute to clinical improvement. Conclusions: CRP response, needed oxygen and OSCI scores can be used as prognostic factors to select patients who will benefit from high-dose corticosteroid pulse therapy.