In recognizing that writing is a social act, collaborative writing has received increased attention in second language (L2) classrooms. A large body of research literature explores the varied ways of L2 collaborative writing development in tertiary education settings, but relatively little is known about the implementation of collaborative writing in K-12 classrooms. In this study, the authors systematically reviewed a total of 12 peer-reviewed empirical studies on the use of collaborative writing in K-12 L2 classrooms to provide new insights into this particular context. Comprehensive analysis has been conducted, including the research context, writing task, mode of interaction, research focus, and assessment. The findings reveal that the majority of the selected studies have involved K-12 students learning English as a target language with diverse writing tasks. Three different collaborative writing approaches have been implemented in K-12 classrooms of L2, including face-to-face, online, and a blend of both styles. The main research foci of the reviewed literature are writing processes, writing outcomes, and collaborative writing affordances. Diverse assessment methods are utilized to measure L2 writers' writing process and product. Drawing on the analyses, the researchers discuss the pedagogical implications and research strands that deserve further examination.