Background: Hospital beds are among valuable resources for care delivery. Therefore, optimum use of them is crucial for increasing the efficiency of health-care services and controlling health-care costs. Objective: This study intended to evaluate inappropriate patient stay (IPS) in hospital settings and its reasons based on the appropriateness evaluation protocol. Methods: This cross-sectional study was conducted on 335 patients hospitalized in a tertiary care university hospital. Data were gathered prospectively by 13 hospital nurses during a 6-month period. IPS rate was evaluated using a checklist, the 27 criteria of which were related to medical services, nursing/life support services, and patient's conditions. Moreover, a 12-item checklist was used to determine physician-, hospital-, and patient/family-related factors behind inappropriate hospital stay. Results: In total, 121 of 1925 (6.3%) hospitalization days of 335 patients were determined to be inappropriate. Neurosurgery and gynecology wards had the highest and the lowest inappropriate hospital stay rates (22.5% vs. 0%), respectively. The main reasons behind inappropriate hospital stay were hospital-related factors (33.1%), physician-related factors (29.1%), and patient-related factors (21.3%). Conclusion: A wide variety of physician-, hospital-, and patient/family-related factors contribute to IPS. Given the multifactorial causes of IPS, reducing its rate necessitates multidisciplinary approaches.