The purpose of this study was to assess the prevalence and characteristics of HIV drug resistance mutations among antiretroviral therapy (ART)-naive and ART-experienced patients in South Korea. A total of 50 ART-naive and 34 ART-experienced Korean HIV-1-infected patients who visited an urban hospital from February 2007 to March 2011 were included. Most patients (86.9%) were infected with clade B HIV-1. Six (12%) ART-naive and 22 (64.7%) ART-experienced patients had HIV strains with resistance mutations. Among ART-naive patients, V179D was the most common mutation, being found in five ART-naive patients. Among ART-experienced patients, M184V was the most common mutation. Eight of 34 ART-experienced patients had thymidine analogue mutations (TAMs). The prevalence of drug-resistant HIV-1 in ART-naive patients was higher than in previous reports, and 50% of patients with virologic failure harbored strains with multiple resistance mutations. HIV drug resistance testing should be recommended to guide therapy of ART-naive patients in South Korea.