This study investigates the role of motivation and learner variables in bilingual vocabulary development among first language (L1) Japanese students attending hoshuukoo (i.e., supplementary academic schools for Japanese-speaking children) in the United States. One hundred sixteen high school students ages 15-18 from eight hoshuukoo completed Japanese and English vocabulary tests and a student characteristics questionnaire. A principal component analysis identified six motivational factors: preference for Japan, Japanese heritage, no choice, career orientation, American identity, and positive perceptions. The students highly rated career orientation, positive perceptions, and Japanese heritage as reasons to learn both languages. Regression analysis revealed negative effects of American identity and career orientation on L1 Japanese vocabulary knowledge and a positive impact of positive perceptions on second language (L2) English proficiency. The students' college plans were highly related to the motivational factor of preference for Japan. In sum, bilingual competence can be related to motivational factors, and individual differences in bilingual proficiency among young learners of Japanese as a heritage language can be, at least partially, accounted for by socio-psychological factors.