The effect that online extensive reading (ER) of English texts has on learners' reading comprehension and motivation to read is not well understood. The purpose of this systematic review was to examine the efficacy of online ER by examining quantitative and mixed-methods studies that examined the effect of online ER on learners' reading achievement, motivation to read, and other perceptual and emotional aspects of English language proficiency. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria were followed in this study. Online ER-related articles were selected based on the predefined eligibility criteria. Then, the 31 English-language online ER studies published between 2010 and 2021 were screened and extracted from electronic databases. The screened studies were those that involved students above 15 years of age. The findings indicated that online ER activities could be used in two ways. First, teachers could incorporate online reading strategies and technological tools for online ER to help students improve their reading achievement. Second, if they wanted to increase their students' motivation to read, they could emphasize the use of online reading management systems and reading resources for ER. Additionally, the study discovered that the size of the vocabulary and the volume of reading were predictors of online ER's success. Additionally, the article discusses the review's implications.