Mathematical critical thinking is one of the essential competencies students must possess to face the challenges of the 21st century. However, the 2022 PISA results show that Indonesian students’ mathematical abilities are still relatively low. A preliminary study at SMP Negeri 3 Kuningan also revealed low levels of students’ critical thinking skills, as indicated by difficulties in analyzing and solving mathematical problems. This study aims to examine the effectiveness of Math Adventure media with a STEM (Science, Technology, Engineering, and Mathematics) approach on students’ mathematical critical thinking skills. The research employed a quantitative method with a quasi-experimental design of the non-equivalent control group type. The sampling technique used was purposive sampling, involving two classes as samples: the experimental class using Math Adventure based on STEM, and the control class using conventional learning methods. Research instruments consisted of tests (pretest and posttest) and a student response questionnaire. Data analysis was conducted using normality tests, homogeneity tests, t-tests, and N-gain analysis. The results showed a significant difference and improvement in students’ mathematical critical thinking in the experimental class. Student responses to the learning media were also very positive. Thus, Math Adventure media with a STEM approach is effective in improving students’ mathematical critical thinking skills.