The dynamics of students' engagement in online mathematics learning during the pandemic have been completely distinctive from face-to-face learning. To further the investigation, the current study aims to examine the relationship between student adaptability and engagement and the mediating role of self-regulation and grade level, parental education level, student age, and student gender. A total of 339 students, with an average age of 11.16 years, from three public elementary schools in Yogyakarta, Indonesia, participated in this study. The findings demonstrated that: 1) adaptability has a positive and significant effect on student self-regulation, 2) self-regulation has a positive and significant effect on student engagement in online mathematics learning, and 3) adaptability has a significant positive influence on student engagement in both directly and through the mediation of student self-regulation. The study has a significant implication for the student learning environment, especially parental involvement. Recommendations to encourage parental involvement for online learning engagement are made.