Despite the known roles of working memory and background knowledge in the process of reading comprehension, few attempts have been made to elucidate the interaction between these two individual difference factors in second language (L2) reading comprehension. The current study investigated whether and how working memory and background knowledge combine to facilitate L2 reading comprehension in a context where L2 students encounter texts with and without relevant background knowledge. Seventy-nine adult Korean learners of English as a foreign language participated and completed tasks for working memory, L2 knowledge, and L2 reading comprehension. The results revealed that L2 readers with higher working memory capacity benefitted more from the provision of background knowledge, which led to achieving better reading comprehension than readers with low working memory. This finding highlights the role of working memory in L2 reading in terms of using existing resources to one's advantage particularly because no significant difference on L2 measure scores was found between the high- and low-working memory groups. Pedagogical implications are discussed regarding the importance of not only providing background knowledge when it is not already present but also following up with explicit instructional support to help all readers utilize what is available to them.