Objective: This study aims to validate the Korean version of Familial Ethnic Socialization Measure (K-FESM) for 5th and 6th grade multicultural adolescents of marriage migrant mothers from China and Vietnam in Korea. Method: The sample was 358 adolescents who have Korean fathers and marriage migrant mothers (mother’s country of origin: 157 Vietnam, 105 China, 56 Philippines, 40 Japan). The original FESM has 12 items but in K-FESM 3 items were replaced in consideration of unique characteristics of multicultural families in Korea. Analyses including internal consistency analyses, principal-component, exploratory and confirmatory factor analyses, and criterion-related validity tests were conducted for 4 groups by the country of origin of participants’ mothers. Results: The results of internal consistency were good for each group. Adolescents with marriage migrant mothers from Philippines and Japan, however, were found to be inappropriate for factor analyses. Factor analyses showed that the one-factor model was suitable for adolescents of marriage migrant mothers from Vietnam and China separately. Criterion-related validity test using correlations between familial ethnic socialization and ethnic affirmation/belonging confirmed the validity of K-FESM. Conclusions: The Korean version of FESM was found to be a reliable and valid tool for measuring ethnic socialization for multicultural adolescents of marriage migrant mothers in Korea. Using K-FESM, research on adolescents in multicultural families could be expanded and improved.