Abstract
This study presents the development of a supplementary geometry book for elementary students by integrating Reog Ponorogo ethnomathematics into the Realistic Mathematics Education (RME) approach to enhance numeracy literacy. The research employed a Research and Development (R&D) methodology using the ADDIE model (Analysis, Design, Development, Implementation, Evaluation). Validation instruments involved expert evaluations in content, pedagogy, media, and language, alongside practicality questionnaires and numeracy literacy assessments. The developed book achieved high validity across expert domains: 89.8% (content), 91% (media), 94% (learning), and 90% (language). Student response also indicated high practicality (82.3%). A one-sample t-test and N-gain analysis showed a significant improvement in numeracy literacy, with an N-gain score of 0.6461 (categorized as moderate), reflecting a “quite effective” level. These findings suggest that incorporating ethnomathematical elements from local culture within an RME framework can provide meaningful, contextual learning experiences and foster mathematical understanding in early-grade students.