Purpose: Despite the importance of hospital bed network during the pandemic, there are scarce data available regarding factors predictive of prolonged length of hospitalization of COVID-19 patients. Methods: We retrospectively analyzed a total of 5959 consecutive hospitalized COVID-19 patients in period 3/2020-6/2021 from a single tertiary-level institution. Prolonged hospitalization was defined as hospital stay > 21 days to account for mandatory isolation period in immunocompromised patients. Results: Median length of hospital stay was 10 days. A total of 799 (13.4%) patients required prolonged hospitalization. Factors that remained independently associated with prolonged hospitalization in multivariate analysis were severe or critical COVID-19 and worse functional status at the time of hospital admission, referral from other institutions, acute neurological, acute surgical and social indications for admission vs admission indication of COVID-19 pneumonia, obesity, chronic liver disease, hematological malignancy, transplanted organ, occurrence of venous thromboembolism, occurrence of bacterial sepsis and occurrence of Clostridioides difficile infection during hospitalization. Patients requiring prolonged hospitalization experienced higher post-hospital discharge mortality (HR = 2.87, P < 0.001). Conclusions: Not only severity of COVID-19 clinical presentation but also worse functional status, referral from other hospitals, certain indications for admission, certain chronic comorbidities, and complications that arise during hospital stay independently reflect on the need of prolonged hospitalization. Development of specific measures aimed at improvement of functional status and prevention of complications might reduce the length of hospitalization.