Relatively little is known about the role of ambiguity in adult second-language learning. In this study, native English speakers learned Dutch-English translation pairs that either mapped in a one-to-one fashion (unambiguous items) in that a Dutch word uniquely corresponded to one English word, or mapped in a one-to-many fashion (ambiguous items), with two Dutch translations corresponding to a single English word. These two Dutch translations could function as exact synonyms, corresponding to a single meaning, or could correspond to different meanings of an ambiguous English word (e.g., wisselgeld denotes the monetary meaning of the word change, and verandering denotes alteration). Several immediate and delayed tests revealed that such translation ambiguity creates a challenge for learners. Furthermore, words with multiple translations corresponding to the same meaning are more difficult to learn than words with multiple translations corresponding to multiple meanings, suggesting that a one-to-many mapping underlies this ambiguity disadvantage.