The Genetic Counseling Outcome Scale (GCOS-24) was developed to measure patient-reported outcomes to evaluate the effectiveness of genetic counseling and testing services. In the current study, the Korean version of GCOS (K-GCOS) was developed to reflect the sociocultural characteristics of Korea, and its clinical applicability was assessed. Overall, 231 Koreans, including patients with genetic diseases and their family members, participated and completed the K-GCOS, Hospital Anxiety and Depression Scale (HADS), Multidimensional Health Locus of Control (MHLC) scale, and Satisfaction with Life Scale (SWLS). Validity was examined by assessing the correlations between K-GCOS scores and other relevant scale scores. Reliability was confirmed using Cronbach's alpha and test-retest scores, measured over 2 weeks. We performed exploratory factor analysis of the five structures of GCOS-24. For K-GCOS, four-factor structures were identified: "cognitive-behavioral control," "uncertainty about control," "hope," and "emotional regulation." Four original GCOS-24 items were removed because of low factor loadings and small inter-item correlations. K-GCOS-20 scores were positively correlated with SWLS (r = 0.456) and MHLC-internal (r = 0.213) scores but negatively correlated with HADS (anxiety r = -0.428, depression r = -0.469) and MHLC-internal (r = -0.278) scores. These findings demonstrate that K-GCOS-20 is a reliable and valid tool for evaluating genetic counseling services in Korea.