The research aims to develop an instructional tool based on Discovery Learning (DL) combined with a Metacognitive Knowledge Strategy (MKS) to enhance students' Critical Thinking Skills (CTSs). In doing so, the study employed a Research and Development (R&D) method to develop such a tool. The developed instructional tool was tested for its validity by experts and practitioners' evaluation. Further, the empirical data were collected from the results of implementation in learning and the student's responses, while the data of tools effectiveness were acquired from the critical thinking tests given to students (analyzed by related t-test). The developed instructional tools were implemented in a limited-scale trial of 32 students and a large-scale trial of 59 students. The results show that: firstly, the DL and MKS-integrated instructional tools are stated as valid in terms of the lesson plan, student worksheet, and critical thinking test. Secondly, the practicality criteria have been successfully met; the learning implementation, students' activity, and students' responses were regarded as in accordance with the feasibility standard. Thirdly, the instructional tool was deemed effective in enhancing students' CTSs (p = 0.05).