Background: Polypharmacy, the use of multiple drugs, is a growing concern in older adults. It has been reported that the prevalence of polypharmacy in elderly patients is higher in Korea than in other countries. The objective of this study was to investigate the prevalence and associated factors of polypharmacy in elderly outpatients who visited a Seoul Medical Center. Methods: We retrospectively reviewed medical records of 12,551 elderly patients aged ≥65 years who visited outpatient clinics in a public hospital in Seoul between January 1, 2018 and December 31, 2018. We defined “polypharmacy” as the use of ≥6 medications per person; “major polypharmacy,” ≥11 medications per person; and “excessive polypharmacy,” ≥21 medications per person. Multivariate logistic regression analysis was performed to estimate odds ratios (ORs) and 95% confidence intervals (CIs) to investigate the risk factors for polypharmacy. Results: Among the elderly outpatients studied, 40.7% had polypharmacy, 11.9% major polypharmacy, and 0.8% excessive polypharmacy. The lowerincome medical aid population is associated with polypharmacy (OR, 1.70; 95% CI, 1.47–1.97). Conclusion We observed a high prevalence of polypharmacy in elderly outpatients, especially the medical aid population, who visited a Korean public hospital. Nationwide vigorous efforts to assess and reduce the prevalence of polypharmacy are urgently required for the Korean older population.