The present study aimed to examine the relationships between reading and writing, and the role of L2 proficiency in moderating these relationships, among 8th and 11th graders in South Korea. A total of 257 eighth and 322 eleventh graders learning English as a foreign language participated in Study 1 and Study 2, respectively. The findings indicated that (1) reading and writing have a two-way relationship regardless of grade levels, (2) a two-way relationship between reading and writing exists not only within L2 but also across L1 and L2 (L1 reading [right arrow] L2 writing, L1 writing [right arrow] L2 reading), and (3) the strength of the relationship between reading and writing is moderated by the level of L2 proficiency. The authors discuss these results in terms of current research on reading-writing relationships and provide implications for pedagogy and future research.