Research has indicated the importance of vocabulary knowledge and background knowledge in second/foreign language (L2) listening comprehension. However, previous studies treated these two factors separately, and no study has examined the simultaneous contribution of these two factors to L2 listening comprehension. Therefore, the aim of this study is to examine whether background knowledge moderates the relationship between vocabulary knowledge and L2 listening comprehension. To this end, one-hundred and fifty-one L2 learners participated in this study and completed instruments measuring constructs of vocabulary knowledge (breadth and depth of vocabulary knowledge), background knowledge, and listening comprehension. Results of the study indicated that, first, measures of both breadth and depth of vocabulary knowledge contributed to the listening comprehension while vocabulary breadth was a stronger predictor. Second, findings of the moderation analysis revealed that background knowledge moderated the relationship between vocabulary knowledge and listening comprehension. These findings suggest that background knowledge can help language learners to have a better performance in listening comprehension tests only if they have high levels of vocabulary knowledge.