The widespread use of large language models, such as ChatGPT, has changed the learning behaviours of undergraduate students, raising issues of academic dishonesty. This study investigates the factors that influence the ethical use of ChatGPT among undergraduates using the recently proposed Theory of Reasoned Goal Pursuit. Through a qualitative elicitation procedure, 26 salient beliefs were identified, representing procurement and approval goals, advantages and disadvantages, the social influence, and factors facilitating and hindering ethical use of ChatGPT. A subsequent, two-wave quantitative survey provided promising evidence for the theory, revealing that positive attitudes and subjective norms emerged as key antecedents of motivation to use ChatGPT ethically.