This study investigated how word and child characteristics affect children's ability to learn the meanings of novel words. Participants were fourth- and fifth-graders representing native English speakers (NE) and bilingual learners with fluent English proficiency (FEP) and designated English Learners (EL). Students were taught the meanings of a series of novel words that were either morphologically related or unrelated. Results showed that compared to the EL group, the NE and FEP groups were better able to learn the meanings of morphologically related words than morphologically unrelated words. The effect of morphological relatedness on semantic learning was stronger for the target words with familiar suffixes than with novel suffixes. Students learned the meanings of derivatives with familiar suffixes faster than the derivatives with novel suffixes. Verbal working memory and word reading fluency significantly predicted word learning performance. Implications for vocabulary instruction for learners with varying English proficiency are discussed.