The purpose of the current study was to explore longitudinally the bidirectional cross-language effects between morphological awareness and vocabulary development across English and French over one year. Participants included 81 Grade 1 students and 75 Grade 2 students. All were non-native speakers of French, and demonstrated average English word reading and vocabulary skills. They received formal instruction entirely in French at school. Children were tested at two time points spaced one year apart with tasks tapping inflectional and derivational morphological awareness, and vocabulary. They also completed measures of phonological awareness, word reading, and non-verbal reasoning. The Grade 1 students were assessed in English only at pretest, and in English and French at post-test. The Grade 2 students were assessed in English and French at both time points. As predicted, regression analyses revealed that English inflectional awareness predicted unique variance in French vocabulary among Grade 1 students. Among Grade 2 students, cross-lagged analyses showed that both English inflectional and English derivational awareness predicted gains in French vocabulary between Grades 2 and 3. Conversely, counter to our expectations, French morphological awareness did not predict gains in English vocabulary over time. Taken together, these results extend the extant corpus of research to substantiate that children can leverage morphological awareness from their primary language to facilitate their vocabulary learning in their second language.