This research aims to develop and assess the feasibility of project-based Indonesian language textbooks in enhancing critical thinking among Medan State University students. Following the 4-D model of Research & Development, the process involves analysis, textbook design, material development, and dissemination. Needs analysis includes front-end and material design analyses to formulate learning objectives. The design stage entails compiling project-based learning materials, selecting media, and designing the book's specifications. In the development stage, textbooks are produced and validated by experts, with validation results exceeding 87% for both material and media. Dissemination involves distributing the textbooks to students to test practicality, resulting in a practicality test score of 87.74%. Validation and implementation scores indicate "very feasible" status without the need for revision. Consequently, the project-based Indonesian language course innovation is deemed "valid" and eligible for use as the primary teaching material at Medan State University, promising to enhance students' critical thinking skills.