This comparative study investigates the motivation underlying second language (L2) Korean and third language (L3) English learning among immigrant children from multicultural families in South Korea. Utilizing a questionnaire developed by Jeon and Kim (2015), 56 participants provided their status of motivational constructs, including ideal L2/L3 self, ought-to L2/L3 self, instrumental motivation, motivated behavior, attitudes towards target languages, and integrative orientation. The results revealed divergent motivational patterns for L2 and L3 learning, influenced by factors such as gender, grade, and linguistic proximity. Further, integrative orientation surfaced as a crucial predictor for children’s intended learning efforts in both L2 and L3. Interviews with teachers who taught the participants L2 Korean and L3 English also supported the quantitative results. In accordance with previous studies, these findings underscored the dynamic nature of foreign language learners’ motivation. Taken together, this study suggests recognizing the distinct attributes of each motivational system and devising pertinent motivational strategies for L2 and L3 teaching.