Research has established the link between socioeconomic status (SES) and children's reading development. However, the theoretical mechanisms underlying the association between them have yet to be fully understood. Moreover, few previous studies exploring the mechanisms tapped both first language (L1) and second language (L2) at the same time. In this study, we examined the direct and indirect relationships between SES and reading comprehension in both Chinese (L1) and English (L2) among Hong Kong Chinese children. Metalinguistic awareness, word reading, and vocabulary were posited as potential mediators. Participants were 184 Hong Kong Chinese students in Grades 3 and 4. A one-year longitudinal study was conducted with data collected across two time points. Path analysis revealed that SES exerted significant direct and indirect effects on reading comprehension "via" metalinguistic awareness in both languages after controlling for age and non-verbal intelligence. Moreover, for Chinese, SES had significant indirect effects on reading comprehension via vocabulary but not word reading. In contrast, for English, SES had a significant indirect effect on reading comprehension via word reading but not vocabulary. Theoretically, the findings shed light on the potential mechanisms underlying the relationship between SES and reading comprehension in both languages. Educational implications are also discussed.