This study used a novel word learning paradigm to investigate the role of morphology in the acquisition of complex words, when participants have no prior lexical knowledge of the embedded morphemic constituents. The influence of morphological family size on novel word learning was examined by comparing novel stems (torb) combined with large morphological families (e.g., torbnel, torbilm, torbla, torbiph) as opposed to small morphological families (e.g., torbilm, torbla). In two online experiments, participants learned complex novel words by associating words with pictures. Following training, participants performed a recognition and a spelling task where they were exposed to novel words that either did or did not contain a trained morpheme. As predicted, items consisting of a trained and an untrained constituent were harder to reject but easier to spell than those that did not contain any trained constituents. Moreover, novel words including trained constituents with large morphological families were harder to reject than those including constituents with small morphological families. The findings suggest that participants acquired novel morphemic constituents without prior knowledge of the constituents and point to the important facilitatory role of morphological family size in novel word learning.