Although writing instruction can positively impact reading for students across grades and levels of literacy, the extent to which these findings generalize to young students with literacy difficulties is unclear due to the dynamic nature of reading-writing relations. The purpose of this meta-analysis was to examine the effects of writing instruction on the reading outcomes of students in grades pre-K-5 who have reading, writing, or co-occurring reading and writing difficulties. Across 19 studies and 72 effects, writing instruction had a positive effect on reading outcomes in English (g = 0.27, 95% CI [0.13, 0.41], p < 0.01). Descriptively different subset effects for higher-intensity instruction (small student group, greater total hours) could not be reliably estimated. Effects were moderated by the focus of instruction, with transcription instruction associated with larger effects. Percentage of instructional time spent writing and type of comparison condition (reading treatment or control) did not moderate effects. Implications for the design of early writing interventions are discussed.