Background: Immersive virtual reality (IVR) applications that support student learning have gained increasing interest. However, empirical studies exploring the educational potential of using IVR in primary school science classrooms are lacking. Objectives: This study developed a series of IVR science lessons for primary school students and examined the effects of these lessons on learning outcomes. Methods: Our mixed-method approach employed pre- and post-tests to measure academic achievement, questionnaires to measure motivation and cognitive load, and semi-structured interviews to further explore students' feelings and attitudes about IVR science lessons. Participants included 362 Grade 4 students assigned to either the experimental (IVR-based classroom) or control (traditional classroom) groups. The experimental group engaged in IVR science lessons using Head-Mounted Displays; the control group learned the same material through traditional methods. Results and conclusions: The results showed that the IVR-based classroom significantly improved primary students' academic achievement and science motivation and decreased their cognitive load. Moreover, the experimental group reported positive experiences with the IVR science lessons. Takeaways: These findings help unpack the relationships between IVR and academic achievement, science motivation, and cognitive load in primary school science lessons, thus providing insight on integrating IVR into existing classrooms.