Blended synchronous learning enables online learners to participate in class activities from geographically separated sites. Due to various challenges, however, online learners are often harder to be engaged and their engagement levels are lower than that of classroom counterparts. This review summarized and synthesized the challenges that led to the low engagement of online learners and the strategies applied to address the challenges and, thus, increased learner engagement in the empirical studies published in recent years. A total of 33 articles were systematically selected and analyzed. By following the constant comparative analysis approach, this article categorized the challenges into four main themes: (1) inadequate online learner-instructor interactions, (2) insufficient online learner-classroom learner interactions, (3) limited online learner-content interactions, and (4) technological constraints. Meanwhile, this article grouped the strategies into five main themes: (1) initiating frequent interactions, (2) letting online learners play active roles, (3) applying continuous assessment, (4) optimizing technological conditions, and (5) professional development. This article further compared subchallenges and substrategies to identify, which strategies could effectively address each challenge. Implications for instructors, learners, and administrators are discussed, and future research areas are suggested.