Word learning in a second language (L2) is a complex process, which is affected by learner-related (e.g., morphological awareness) and language-related (e.g., word semantic transparency) factors. Morphological awareness is learners' sensitivity to the morphological structure of printed words, and semantic transparency is the degree to which word-internal morphemes contribute to the whole words' meanings. However, little is known regarding how these morpheme-related factors jointly function in L2 word learning. Thus, this study focuses on how learners' morphological awareness and word semantic transparency affect word-meaning retention in L2 Chinese, particularly the extent to which semantic transparency shapes the contribution of learners' morphological awareness. Thirty-four L2 Chinese learners from an American university participated in this study. All the participants received a 40-min learning session and five paper--pencil measures. The main findings showed that (1) L2 learners' morphological awareness contributed to their word-meaning retention beyond L2 linguistic knowledge; (2) L2 learners performed better when recalling semantically transparent words than opaque words after learning sessions; and more importantly (3) learners with higher morphological awareness performed better than those with lower morphological awareness when recalling semantically transparent words, but the learners in both groups had similar performances when recalling opaque words.