This study investigated the concurrent contributions of three components of metalinguistic awareness (i.e., phonological awareness, morphological awareness, and syntactic awareness) to the writing competence of primary three English-Chinese bilingual children in Singapore (n = 390) and monolingual Chinese-speaking children in Mainland China (n = 190). Hierarchical regression analyses found that the three components of metalinguistic awareness differed in their contributions to writing competence across the languages and between the two groups of children, with morphological awareness and syntactic awareness explaining markedly more variance than phonological awareness. Furthermore, SEM results revealed a robust cross-linguistic association between English and Chinese metalinguistic awareness in the bilingual children, which appeared to jointly undergird and support writing competence in both languages. Finally, home language use significantly predicted not only the bilingual children's English metalinguistic awareness but also their writing performance in Chinese. These findings lend support to Cummins's Common Underlying Proficiency and Developmental Interdependence Hypotheses, and underscore the importance of language context, formal instruction, and language features in children's literacy development.