Background: The prevalence and clinical characteristics of psoriatic arthritis (PsA) in patients with psoriasis are not well described in Asian populations, including Koreans. Objective: The purpose of this study was to investigate the prevalence of PsA by using the classification of psoriatic arthritis (CASPAR) criteria on the basis of physical examination only, as well as its correlation with psoriasis severity and other medical conditions including nail psoriasis. Methods: A single-center, cross-sectional observational cohort study was conducted, and the included patients were evaluated for PsA according to the CASPAR criteria. The psoriasis area severity index (PASI) and the nail psoriasis severity index (NAPSI) were calculated. Results: The prevalence of PsA in patients with psoriasis in Korea was 13.5%. When performing logistic regression, hyperlipidemia and localized pustular psoriasis were found to be significant predictors of PsA. The PASI score was significantly higher in PsA patients than in those with psoriasis alone (p=0.014). Psoriatic nail involvement was found in 85.5% of the study population, and all PsA patients had nail psoriasis. The mean NAPSI score was higher in patients with PsA; however, the difference was not statistically significant. Conclusion: There was a close relation between psoriasis severity and PsA, although nail psoriasis severity was not related to PsA status. Dermatologists can diagnose PsA from current physical findings by using the CASPAR criteria. To validate the CASPAR criteria for PsA diagnosis, the definition of nail psoriasis clinical types and severity in the CASPAR criteria should be reviewed again.