A yearlong classroom-based intervention was designed to explore the reading-writing connection in second-language literacy by examining whether the development of reading improves writing and vice versa. Middle school learners of English as a foreign language (N = 300) in South Korea were assigned to three treatments that involved extensive reading, extensive writing, or extended regular instruction (serving as a control) as part of their regular school English curriculum once per week for the whole academic year. Pre- and posttests measured changes in reading comprehension and descriptive writing performance. Results indicated that for reading comprehension, all three groups significantly improved over time, but the groups did not differ statistically. As for writing performance, only the extensive reading and extensive writing groups, not the control group, experienced significant gains over time, with the two experimental groups showing no statistically significant differences from each other. Additionally, for reading comprehension, students showed improvement regardless of their English-proficiency levels, whereas only the students with higher proficiency experienced a significant improvement in writing performance. These results suggest that an individual can learn to read by writing as well as by reading and can learn to write by reading as well as by writing, as reading and writing involve some of the same subprocesses. Yet, for writing, the reading-writing connection seems more evident for those second-language learners whose language proficiency exceeds a certain linguistic threshold. For those who have relatively lower language proficiency, the development of writing skills may not proceed as easily, through either extensive reading or extensive writing.