We conducted a meta-analysis to investigate reading-writing relations. Beyond the overall relation, we systematically investigated moderation of the relation as a function of linguistic grain size (word reading and spelling versus reading comprehension and written composition), measurement of reading comprehension (e.g., multiple choice, open-ended, cloze), and written composition (e.g., writing quality, writing productivity, writing fluency, writing syntax), and developmental phase of reading and writing (grade levels as a proxy). A total of 395 studies (k = 2,265, N = 120,669) met inclusion criteria. Overall, reading and writing were strongly related (r = 0.72). However, the relation differed depending on the subskills of reading and writing such that word reading and spelling were strongly related (r = 0.82) whereas reading comprehension and written composition were moderately related (r = 0.44). In addition, the word reading-spelling relation was stronger for primary-grade students (r = 0.82) than for university students/adults (r = 0.69). The relation of reading comprehension with written composition differed depending on measurement of reading comprehension and written composition--reading comprehension measured by multiple choice and open-ended tasks had a stronger relation with writing quality than reading comprehension measured by oral retell tasks; and reading comprehension had moderate relations with writing quality, writing vocabulary, writing syntax, and writing conventions but had weak relations with writing productivity and writing fluency. Relations tended to be stronger when reliability was higher, and the relation between word reading and spelling was stronger for alphabetic languages (r = 0.83) than for Chinese (r = 0.71). These results add important nuances about the nature of relations between reading and writing.