Due to the sudden transition to online education because of the outbreak of COVID-19, various strategies and factors have been extensively investigated to increase the effectiveness and quality of online education. In this study, we reviewed and synthesized the factors correlating with students' online learning satisfaction, considered a basic component when evaluating online education quality. From 42 studies that were systematically searched from nine databases, 164 separated correlates of students' satisfaction were found. These were categorized into four domains: student-related factors, instructor-related factors, quality of online classes, and e-learning readiness. Overall effect size (ES) and four subgroup ESs were then calculated. The results indicated that the domain of instructor-related factors showed the highest ES compared to other domains. Among the factors, instructional support, technology acceptance, and perceived presence were the three top factors exhibiting large ES. Several implications are discussed to enhance these crucial factors.