This study investigated the role of parental support and selected family variables in the first (L1) and second language (L2) vocabulary development of Japanese heritage language (JHL) high school students in the United States. Eighty-two JHL students ages 15-18 from eight hoshuukoo (i.e., supplementary academic schools for Japanese-speaking children) were assessed for their L1 Japanese and L2 English vocabulary knowledge. Simultaneously, the students' parents, at least one of whom was a Japanese native, completed a family background questionnaire. An exploratory factor analysis identified five components that the parents considered effective for supporting their children's language development: Japanese pop culture, reading for pleasure in Japanese, use of Japanese at home, reading to the child, and family conversation. Regression analyses revealed that reading for pleasure in Japanese positively predicted the students' L1 Japanese vocabulary and that Japanese pop culture negatively predicted their L2 English vocabulary. L1 Japanese vocabulary was also predicted by students' dominant language and parents' goals for their children's achievement, whereas L2 English vocabulary was predicted by future residency plan, mother's self-rated English proficiency, and home language use. In short, JHL students' L1 Japanese and L2 English vocabulary development involves different family factors. Consequently, different approaches should be recommended to support HL students' L1 and L2 development.