Working memory is believed to interact with a second language (L2) learning at the cognitive level. The present study sought to explore the impact of L2 readers’ prior knowledge on the contribution of working memory to reading comprehension. Eighty Iranian English learners were divided into two groups of high and low by their scores on L2 knowledge and the topic knowledge tests. Their working memory spans, and reading comprehension abilities were measured via a working memory test and a reading comprehension test respectively. The results indicated that working memory significantly predicted L2 reading comprehension only when the readers had sufficient topic knowledge. The results also show that the learners’ comprehension was mostly determined by their L2 linguistic knowledge, even when they had considerable working memory capacity. The findings imply that readers’ prior knowledge could moderate the contribution of working memory in L2 reading comprehension. A certain level of knowledge in the target language and on the topic is required for L2 readers to help working memory work more efficiently.
