The dynamics of students' engagement in online mathematics learning during the pandemic have differed significantly from face-to-face learning. To further investigate this, the current study aims to examine the relationship between student adaptability and engagement, taking into account the mediating role of self-regulation and the influence of 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 of the study revealed the following: 1) adaptability significantly and positively predicts students' self-regulation, 2) in turn, self-regulation significantly and positively predicts student engagement in online mathematics learning, and 3) adaptability has a significant positive impact on student engagement, both directly and through the mediation of student selfregulation. These findings have significant implications for the student learning environment, particularly with regard to parental involvement. Recommendations are provided for creating environmental conditions that promote online learning engagement through adaptability and selfregulation.