This study identified changes in Korean and heritage language proficiencies across five waves (2011-2015) and examined the association between linguistic acculturation trajectories and adjustment (2016) among Korean adolescents with immigrant family backgrounds (N = 1441; 51.21% female; M[subscript age(baseline)] = 9.97). All adolescents were from multiethnic families with 91.65% comprising a non-Korean Asian mother (e.g., Japan, Philippines, China, Thailand) and Korean father. Parallel process latent class growth modeling and multivariate regression analysis were conducted to estimate the trajectories of linguistic acculturation and their effects on adolescents' future adjustment. Linguistic acculturation trajectories were classified into three latent groups. Bilingual adolescents demonstrated better adjustment over adolescents in other trajectories. Findings highlight the importance of considering patterns of change in linguistic acculturation for understanding Korean multicultural adolescents' adjustment.