This study examined the relationship between parental and adolescent eHealth literacy and its impact on online health information seeking. Data were obtained from 1,869 junior high school students and 1,365 parents in Taiwan in 2013. Multivariate analysis results showed that higher levels of parental Internet skill and eHealth literacy were associated with an increase in parental online health information seeking. Parental eHealth literacy, parental active use Internet mediation, adolescent Internet literacy, and health information literacy were all related to adolescent eHealth literacy. Similarly, adolescent Internet/health information literacy, eHealth literacy, and parental active use Internet mediation, and parental online health information seeking were associated with an increase in adolescent online health information seeking. The incorporation of eHealth literacy courses into parenting programs and school education curricula is crucial to promote the eHealth literacy of parents and adolescents.