Although English and German are both Germanic languages, due to various historical changes, many of their cognates are no longer easily recognizable. This study examined whether knowledge of language history can be beneficial to learners when learning English-German cognates. Thirty-five English-speaking second language (L2) learners of 3rd-semester German at an American university were assigned to either an intentional or incidental learning condition. The intentional group received explicit instruction on 2 historical sounds shifts (Second Germanic Sound Shift, Ingvaeonic Palatalization) and relevant historical semantic changes. In contrast, the incidental group carried out various activities that exposed learners to cognates through reading and oral communication tasks. Results indicate that the intentional group significantly outperformed the incidental group, suggesting that knowledge of language history can be beneficial to learners when learning historically related languages. Declarative knowledge of the historical changes led to significantly greater vocabulary gains and it also provided learners with a tool kit to correctly predict the meaning of several cognates they had not previously encountered before. This study has broad implications for vocabulary learning, language teaching, and applied historical linguistics.