Writing is an important mode of thinking and learning for elementary students. Consistent efforts have been made to encourage discipline-specific writing, yet defining qualities of elementary mathematical writing have historically been underdeveloped. This article offers educators a new framework that conceptualizes mathematical writing as writing to reason and to communicate mathematically. Specifically, the framework defines four types of elementary mathematical writing: exploratory, informative/explanatory, argumentative, and mathematically creative. The authors explain and explore these types and their associated purposes through classroom vignettes. Informed by existing practices in mathematics and writing, strategies are offered to support teachers in the implementation of mathematical writing.