Abstract This study examines how ChatGPT influences high school students’ cognitive and behavioral engagement in Physics learning, drawing on survey data from 251 students at Vo Minh Duc High School in Ho Chi Minh City. Although generative AI is increasingly adopted in education, little is known about how Vietnamese students use ChatGPT in a cognitively demanding subject like Physics-representing a notable research gapin a cognitively demanding subject like Physics, which represents a notable research gap. Using a mixed-methods design, the study explores how students employ ChatGPT, how they perceive its usefulness and reliability, and how its use shapes learning behaviors. Findings indicate that students commonly use ChatGPT as a self-study aid and perceive it as helpful for understanding complex concepts, yet many express concerns about accuracy and show signs of overreliance when using the tool without guidance. The study highlights the dual impact of ChatGPT-supporting learning while potentially reducing independent thinking-and contributes evidence potentially reducing independent thinking and contributes evidence to inform AI literacy development, teacher training, and pedagogical frameworks for responsible integration of generative AI in secondary education.