Asynchronous online discussion is a learning activity commonly used in online teaching. The way instructors participate in asynchronous online discussions significantly impacts students' learning outcomes. This systematic review aims to determine the effects of instructor participation in asynchronous online discussions on student performance by analysing 25 relevant studies in the past twelve years (2011-22). The findings show that the research was focused on the presence, frequency and type of instructor participation. The effects of instructor participation on student performance were mixed and concentrated in five main aspects: the quantity of student participation, the quality of student participation, student achievements, student emotions and student learning time. Not only the teaching experience of instructors but also course content, learning objectives, class sizes and learner characteristics influenced the effects. Based on the findings, several recommendations were made to help instructors effectively participate in asynchronous online discussions, thereby contributing to improving the quality of online education.