This study aimed to determine the effectiveness of the Project-Based Learning (PjBL) model with an ethnomathematics-based STEM approach in improving students’ mathematical literacy skills in plane geometry. The study employed a quantitative approach using a pretest-posttest design. The subjects were 28 fifth-grade students at UPTD SD Negeri Kenarilang, selected using saturated sampling. Data were collected using an essay-based mathematical literacy test. Data analysis included descriptive statistics, a normality test, and a mean difference test using a paired-samples t-test. The results showed an increase in students’ mean mathematical literacy score from 45.53 on the pretest to 79.71 on the posttest. The mean difference test yielded a calculated tcount (tcount) of 34.93, which was greater than the critical tvalue (tval) of 1.701 at a significance level of 0.05. The test decision indicated that H? was rejected, demonstrating a significant difference between students’ posttest and pretest mathematical literacy scores. Therefore, the PjBL model with an ethnomathematics-based STEM approach was effective in improving students’ mathematical literacy skills in plane geometry. These findings suggest that integrating project-based learning, STEM, and ethnomathematical contexts can provide a meaningful approach to strengthening mathematical literacy in elementary education.