Gifted learners are usually identified as students who show higher levels of aptitude or competence compared to their peers of the same age. Their advanced abilities often require specially designed educational strategies to meet their intellectual needs. By designing educational strategies that are appropriately challenging and intellectually stimulating, educators can support gifted students in reaching their full potential, ensuring that their talents are well-developed. This study aims to investigate how higher-order thinking skills (HOTs) can be developed in gifted students through their participation in an interdisciplinary project within a differentiated learning environment. The HOTs defined in this study are associated with Bloom's upper-level taxonomy, which includes analysing, evaluating, and creating. The interdisciplinary project was designed using a "backward design" approach, incorporating three subjects: History, Islam, and Moral Education. The study focused on employing differentiated instruction in terms of both content and process. Based on the desired learning outcomes of the three courses, the "big idea" (theme), namely "Independence Commemoration," is created to guide the instructor in designing an interdisciplinary project. A case study was adopted in the current study. The study involved 21 students, aged 15 and 16, enrolled in a high school in Northern Malaysia offering the Advanced Smart Education Program. Data was collected from interdisciplinary project discussions in "Google Docs" and analysed qualitatively using the content analysis method. The depth of students' thinking levels was evaluated using the revised Bloom's taxonomy. The findings show that the interdisciplinary project, implemented in a differentiated learning environment, was less effective in promoting students' HOTs, as most students tended to offer superficial responses. The study discussed the challenges of designing and implementing an interdisciplinary project in a differentiated learning environment. It also outlined the implications for school and classroom practices of such a project. Additionally, the study provided recommendations or solutions to address the problems identified based on the findings.