Writing and mathematics are essential in academic achievement. In the present study, we investigated whether writing skills and mathematics skills are related and if so, whether their relation is moderated by participants' grade level (a proxy for developmental phase), subskills of mathematics and writing skills, and assessment characteristics (normed and standardized nature of mathematics and writing tasks, and reliability of mathematics and writing measures), using a meta-analysis. A total of 211 studies (k = 564, N = 1,207,983) met inclusion criteria. Overall, mathematics and writing were moderately related (r = 0.48). The overall magnitude of their relation differed as a function of grade level such that the relation was stronger for students in primary grade levels compared to those in university and above. When explored by higher order and lower order writing and mathematics skills and grade levels, lower order writing and mathematics (0.59) and higher order writing and mathematics (0.48) had substantial relations in primary grades, whereas higher order writing and mathematics were weakly related in adulthood (0.25). The relation also differed by the reliability of mathematics tasks such that the magnitude was stronger for mathematics tasks with higher reliability. For writing, the relation was stronger for writing tasks that were normed and standardized compared to those that were not. These results confirm not only a mathematics-writing relation but also factors that influence this relation. Future work is warranted to investigate mechanisms of the relation and potential practical implications.