Background: Onychomycosis is the most common nail disorder that is especially prevalent in the older age group. Advanced knee osteoarthritis is another senile disorder that is highly associated with advancing age. However, the association between the knee joint disorder and onychomycosis is largely unknown. Objectives: To investigate the prevalence and risk factors of onychomycosis in patients with knee osteoarthritis. Methods: A prospective study was performed on 520 consecutive patients with osteoarthritis who visited the Department of Orthopedics at SMG-SNU Boramae Medical center from 2018 to 2019. These patients were evaluated for onychomycosis by KOH exam and PCR-based reverse blot hybridization assay (REBA). Information regarding potential predisposing factors for onychomycosis, such as age, weight, comorbidities, and environmental factors was collected. Results: Of the 520 patients with knee osteoarthritis, 436 (83.8%) were female and the mean age was 70.2 years. In total, 308 (59.2%) were diagnosed with onychomycosis based on REBA. Multivariable analysis revealed age, male, and being barefooted as meaningful risk factors of onychomycosis. Conclusion: Onychomycosis was highly prevalent in patients with knee osteoarthritis. Our findings confirmed age as an important risk factor, and males had a greater risk of onychomycosis compared to females. The tendency to be barefooted, which may expose patients to more traumatic events, was also associated with the increased risk of onychomycosis.