The present study aimed to investigate the role of classroom climate and self-regulation in terms of cardiac vagal tone and inhibitory control in primary school students' focused attention. A combination of direct and indirect measures was used to assess classroom climate, sustained and selective attention, and inattention behaviors among 62 first graders. We also registered students' cardiac vagal tone at rest and their levels of inhibitory control in a reaction time task. A single underlying factor combining sustained and selective attention as well as (the lack of) inattention behaviors accounted for students' focused attention. A regression analysis revealed that classroom climate, cardiac vagal tone, and inhibitory control interacted to explain first graders' focused attention. In classrooms with a negative climate, high levels of inhibitory control attenuated the negative effect of low cardiac vagal tone on students' ability to remain focused during classroom activities. Implications for theory and practice are discussed.