This study aims to reveal significant factors affecting the Korean language proficiency of immigrants in Korea by comparing the two immigrant groups that constitute the largest sector of the foreign-born population in Korea, migrant workers and marriage-migrant women. A comprehensive survey was conducted on a total of 136 migrant workers and 136 marriage-migrant women. Statistical analysis was followed adopting stepwise multiple regression on six independent variables (length of residence, intensity of target language use, learning time at institutions, age at arrival, linguistic distance, and education level) and one dependent variable (self-reported language proficiency). Results show that length of residence in Korea and intensity of target language use significantly contributed to the Korean language proficiency of marriage-migrant women, whereas age at arrival, intensity of target language use, and education level played a significant role in the development of language proficiency of migrant workers in Korea. Educational implications are discussed, and suggestions are made for expanding language educational support for migrant workers.