This study investigated the effectiveness of physics instruction using the nature of science (NOS) approach on students' academic achievement and motivational beliefs. A quasi-experimental pretest-posttest design with a control group was employed. The statistical population comprised all tenth-grade female students in Kalaleh, Iran, during the 2021–2022 academic year. A convenience sample of 40 students was selected and randomly assigned to an experimental group (n = 20) and a control group (n = 20). The experimental group received physics instruction based on the NOS approach, while the control group was taught using conventional methods. Data were collected using an academic achievement test (reliability coefficient: 0.73) and the Motivated Strategies for Learning Questionnaire (Pintrich & De Groot, 1990). Analysis of covariance (ANCOVA) revealed that the NOS-based instruction was significantly more effective than the conventional approach in improving both students' academic achievement and their motivational beliefs. Given students' weak motivational beliefs and low performance in learning physics concepts, the integration of the nature of science approach into science education is strongly recommended.