Although there is overwhelming evidence highlighting the foundational role of phonological processing and vocabulary skills in reading, less research has focused on the contribution of these skills to spelling, particularly in African languages. This longitudinal study explored cognitive-linguistic predictors of spelling development in Northern Sotho-English bilingual children. Children were assessed on phonological awareness, phonological working memory, rapid automatized naming, and receptive vocabulary in Grade 2 (Time 1) and on spelling development at the end of Grade 3 (Time 2). Phonological awareness and rapid automatized naming were unique within and cross-language predictors of word spelling in both Northern Sotho and English. This study represents a first attempt to understand the processes that support bilingual spelling development in Northern Sotho children learning English as an additional language. The findings contribute to the body of research investigating spelling development in bilingual children who are learning languages with different orthographical systems. Based on the findings, we recommend that bilingual spelling and phonics instruction explicitly reflect the phonological systems of both languages.