Objectives: Half of all drugs are used irrationally around the world, and about half of patients take their medications correctly. Inappropriate use of drugs leads to antibiotic resistance, medication therapy problems, and an increase in drug costs. This study aimed to assess irrational drug use and its associated factors at the outpatient pharmacy of Debre Markos Referral Hospital. Methods: A hospital-based cross-sectional study was used. Systematic random sampling was used as a sampling technique. The data were collected with a structured checklist. The data were entered into EPI Data Version 3.1 and analyzed using SPSS version 20. Descriptive statistics and logistic regression analysis were used for data analysis. Results: The average number of drugs per patient encounter was 2.14. The prevalence of antibiotics use per encounter was 39.3%. Polypharmacy was detected in 62.2% of prescriptions, and injections were prescribed in 13% of prescriptions. The percentage of drugs prescribed with a generic name was 95.5%. In multivariable logistic regression, comorbidities, professionals’ training, and prescribers’ experience were significantly associated with polypharmacy. Patient age, comorbidity, presence of chronic disease, professionals training, and type of diseases were significantly associated with antibiotic prescribing. Conclusion: Our findings revealed there were practices of polypharmacy and antibiotics overuse. Continuous seminars and training on rational prescribing and periodic prescription surveys are recommended to prevent irrational drug use.