Background: Informal payments, as one of the components of out-of-pocket (OOP) payments, are often a form of corruption and outside the official bill. The purpose of the present study was to investigate the frequency of informal payments for inpatient services and its related factors in Tehran, Iran. Methods: In this cross-sectional study, the samples included 450 patients who were admitted to hospital's clinics affiliated to Shahid Beheshti University of Medical Sciences in Tehran, to receive follow-up and post-surgical care. After obtaining patient consent, data were collected via a 28-item questionnaire included two parts of patient's demographic characteristics and informal payments experience that it's validity and reliability were confirmed. Data were analyzed using descriptive statistics and regression analyses in SPSS 16 software. Results: The findings showed 21.1% of the patients had informal payments, paid voluntarily, mostly in cash, for service's staff, in public hospitals, after discharge and, due to the employees' proper behavior and attention. The average informal payments per patient were 5,304,630 IRR (175.4 USD). There was no significant relationship between the informal payment behavior and the patient demographics characteristics (P > 0.05). Conclusions: Despite the efforts made in the HTP to eliminate informal payments in inpatient services, the goal has not been fully realized. A part of the payments was paid at the provider's request and before the patient's admission that these compulsory informal payments are illegal and unethical and therefore, must be controlled and eradicated. Policymakers can take steps to reduce informal payments by improving the healthcare quality, supervision, setting salary and benefits, enforcing laws, and increasing staff incentives.