Over the past decades, theories of cross-language transfer have widely focused on the proficiency required to transfer a single linguistic modality in one language to that same linguistic modality in another language (e.g., first-language reading to second/foreign-language reading). To move beyond cross-language transfer in a single modality, a new theory, the transfer integration hypothesis proposes that reading and writing are related across learners’ first and second languages. To test the transfer integration hypothesis, this systematic qualitative review synthesizes the findings of peer-reviewed studies on the reading-writing connections across languages. Evidence of cross-language reading-writing connections to support the transfer integration hypothesis was found with much variability resulting from literacy and linguistic factors. These factors include multilinguals’ language experience and grade level, reading and writing measures, language exposure, motivation, language proficiency, and language structure similarities and distance.