This meta-analysis examines the effects of phonics instruction on the decoding skills of students with intellectual disability using a random-effects model. Eight single-case experimental studies and six experimental or quasi-experimental group studies met the inclusion criteria, encompassing a total of 297 participants with intellectual disability. The overall effect of phonics instruction on the decoding skills of persons with intellectual disability was large: g = 1.42 (95% CI: 0.75, 2.10). Single-case studies yielded a larger average effect size (g = 1.94, 95% CI: 1.10, 2.78) than group studies (g = 0.41, 95% CI: 0.13, 0.69). Researcher-designed tests also yielded larger effect sizes than standardized tests. The type of interventionist was not a significant moderator. In the majority of the studies, phonics instruction was carried out using a systematic and direct instruction approach and a one-to-one format. Implications for practice and research are presented, and areas that require further investigation are identified.