This research investigated the effectiveness of Inquiry-Based Learning (IBL) on the development of Higher-Order Thinking Skills (HOTS) and learning achievement among first-grade students at Khoon Kham Primary School during the 2025-2026 academic year, with a focus on the subject of science and the environment. This quasi-experimental study used a One-Group Pretest-Posttest Design with a sample of 18 students. The research instruments included IBL lesson plans and a test to measure HOTS and overall learning achievement scores. Data were analyzed using descriptive statistics (mean, standard deviation) and inferential statistics (Paired Samples t-test). The findings showed: Higher-Order Thinking Skills (HOTS): Students' HOTS scores after the IBL intervention (Mean = 1.83) were significantly higher than their scores before the intervention (Mean = 0.33), with a statistical significance of p < 0.001. This indicates that IBL is effective in developing students' higher-order thinking skills. Overall Learning Achievement: Students' overall test scores after the intervention (Mean = 8.28) were significantly higher than their scores before the intervention (Mean = 4.39), with a statistical significance of p < 0.001. This confirms that IBL promotes students' overall academic achievement. In conclusion, this research confirms that Inquiry-Based Learning has a significant positive impact on improving both higher-order thinking skills and learning achievement for first-grade students in science and the environment. The findings provide valuable information that can be used to guide curriculum and teaching method development to fully promote the cognitive potential of students at the primary school level.