Third language (L3) lexical acquisition is still underexplored. In this article I overview theoretical and empirical evidence on L3 lexical acquisition and the role of cross-linguistic influence (CLI) in learning L3 words. I explain the mechanism of CLI as resulting from language co-activation in the multilingual learner's/user's mind. Consequently, I aim to ground L3 lexical studies in previous research on second language (L2) word processing and learning, and to emphasize the role of cross-linguistic similarity (cognates and false cognates) in the process. While capitalizing upon similarity predominantly facilitates acquiring L2 and L3 words, the precise mechanisms of L3 lexical acquisition are still obscured. It is unclear whether any overlap of an L3 form with the native or L2 form suffices to boost learning, or whether all previous languages influence L3 lexical acquisition cumulatively. To seek answers to this issue, I review empirical evidence for CLI and cross-linguistic similarity in L3 vocabulary acquisition from three research strands: L3 word processing experiments, L3 cognate guessing tasks, and L3 word learning experiments. Overall, this article aims to bridge the gap between psycholinguistic and applied linguistic research on L3 lexical acquisition, and argues for controlling an array of variables modulating research outcomes.