It is pointed out that one of the main problems of online learning environments is determining whether students engage or not. As engagement is a complex and multifaceted concept, researchers have stated that engagement is effected by many factors (environmental conditions and learner characteristics) and changes according to the context. Among these factors, discovering the effects of emotions experienced in learning processes on engagement has drawn attention recently. It is stated that there is an effect of emotions on engagement. However, the mechanisms underlying the effect of emotions experienced in online learning environment on student engagement have not yet been discovered, and what potential mediator and/or moderator variables might explain this effect. Therefore, this study aims to establish whether emotions predicted student engagement through their effect on the online learning readiness, and whether the strength of this effect was a function of metacognition. The study group includes 1601 university students. Data were gathered via online survey. The moderated mediation models were analyzed with PROCESS macro. The results displayed that emotions are powerful predictors of online learning readiness. Online readiness mediated the effect of emotions on student engagement in three models. Metacognition moderated the positive emotions and negative emotions model but did not moderate the exploratory emotions model. As a result; the direct effect of emotions on both readiness and engagement reveal the need for strategies to enable students to experience positive and exploratory emotions and to reduce negative emotions in learning environments. This study reveals that increasing students' positive and exploratory emotions and reducing their negative emotions, ensuring their online learning readiness and high metacognition have an impact on online student engagement. Therefore, with the results of this study, it can be stated that taking into account students' emotions, readiness and metacognition in the design of online learning environment and activities may have a positive effect on student engagement.