This study aimed to examine the impact of Science, Technology, Engineering, and Mathematics (STEM) instruction on strengthening the reasoning skills of Indonesian high school students. The 63 participants from senior high school were divided into two groups: STEM group and Traditional group. The STEM group received STEM instruction with an emphasis on the four STEM subjects. Instruction used inquiry-based learning to facilitate instruction and to expand the connection between the science learning process with technology, engineering, and mathematics. Students in the STEM group were divided into small groups each consisting of 5-6 students with the teacher functioning as a facilitator to help the students in the learning processes. The traditional group received conventional instruction through reading, listening, and discussion. These two groups were taught by the same teacher and learned the same topic. Twenty-four two-tier multiple-choice questions from Lawson's classroom test of scientific reasoning were utilized to assess students' reasoning skills. In conclusion, a statistically significant difference in students' reasoning skills between the STEM group and traditional group was found. Recommendations are offered to maximize STEM instruction and reasoning skills' measurement in future research.