In recent years, mathematical thinking and reasoning have been widely discussed to promote students' abilities to apply mathematical knowledge and ideas in their daily living. However, few studies have investigated the role of self-regulation in relation to reasoning. This study examined the effects of self-regulation processes on student mathematical reasoning and academic achievement. Using a quantitative research design and the PLS-SEM technique, data were collected from 248 private school students in Malaysia. The PLS-SEM results showed that behavioral regulations, including processes of self-observation, self-judgment, and self-reaction, are decisive factors in influencing student academic achievement and student mathematical reasoning ability. The dimensions of motivational regulation, including processes of self-efficacy, task value, and mastery goal orientation, are dominant factors influencing student reasoning ability, followed by cognition regulation, which includes use of elaboration strategy and critical thinking skills. The study also found that cognition regulation is a significant mediator of the relationship between motivational regulation and reasoning ability. Behavioral and cognition regulation processes, as well as students' reasoning ability, are the mediators of motivational regulation on academic achievement. The results of this study suggest that teachers should foster the adoption of self-regulation processes in mathematics learning.