In learning mathematics online, parental involvement and student engagement are very crucial for student learning development. Therefore, the purpose of this study was to examine the relationship between parental involvement and student engagement in online mathematics learning. The participants of this study were 251 students in fourth and fifth grade from five elementary schools in Bantul, Yogyakarta, Indonesia. This study employed a cross-sectional survey to achieve the research objectives. The research instruments used in this study were two sets of questionnaires: on parental involvement and student engagement adapted from previous research. This study's findings indicated a significant relationship between parental involvement and student engagement in the online mathematics learning. The need to maximize the role of parents in learning mathematics online is the key to success in bringing students to be actively involved cognitively, socially, and emotionally.