Objectives: To explore the inpatient medication errors and pharmacist intervention at Ministry of Health Hospital, Riyadh, Saudi Arabia. Methods: This is a 9-month cross-sectional study conducted at a 300-bed public hospital to evaluate pharmacist response and prevention of inpatient medication errors in adult and pediatric patients. There is a medication safety officer in the hospital along with a medication safety committee. The following information on medication errors were documented in the form available at the hospital: patient’s demographic information, sources of medication errors, time of errors, type of errors, description of errors, causes of errors, recommendation to prevent such errors and the outcome of errors. The form was developed by using the National Coordinating Council (NCC) for Medication Error Reporting and Prevention (MERP) system. Results: According to the results, the pharmacist prevented a total of 3089 medication errors within 805 patients. About 3.8 errors per prescription were prevented. Most of the prevention occurred during prescribing stage (705 (99.2%)). Patient-related errors (1564 (50.63%)) and prescriber-related errors (1435 (46.46%)) were the most type of prevented errors. Allergy was the most prevented subtype of errors (560 (91.4%)) followed by patient’s body weight (543 (88.6%)) and prescriber data missing/unclear (347 (56.6%)). Most of the errors that were prevented were near miss (93.3%) followed by 6.3% of the errors that reached the patient but did not cause any harm. The highest percentage with respect to the causes of medication errors was missing clinical information (649 (83.7%)) and miscommunication of drug order (627 (80.9%)). The top 20 medications involved in medication errors were oral and intravenous injections (Paracetamol and enoxaparin injection, respectively). Conclusion: The pharmacist plays a very crucial role in preventing medication errors. In order to prevent medication errors and improve patient outcome, the pharmacist provides education to the healthcare professional about medication safety and establishes the intravenous medication guidelines.