<![CDATA[The low mastery of English vocabulary among junior high school students remains a significant challenge in achieving the learning objectives of the Independent Curriculum. This research aimed to develop and evaluate the effectiveness of game-based learning materials for 8th-grade students at SMP Methodist Pantai Labu. The study followed the ADDIE (Analysis, Design, Development, Implementation, Evaluation) model of Research and Development (R&D). Data were collected through expert validation, observations, and a one-group pre-test–post-test design involving 32 students. The findings indicated that the developed materials were highly valid and feasible based on expert judgment. Statistical analysis through the Paired Sample T-Test revealed a significant improvement in students' vocabulary mastery, with the mean score increasing from 57.09 to 83.53 (p = 0.000). The intervention achieved a Large Effect Size (Cohen’s (d = 2.11) and a Moderate N-Gain score (0.63), demonstrating high learning efficiency. Furthermore, the classical mastery rate increased dramatically from 0% to 90.6%, proving that the product effectively narrowed the achievement gap and supported differentiated learning. The integration of gamification elements successfully created a student-centred environment that reduced learning anxiety. It is concluded that game-based learning materials are a robust and effective solution for enhancing English vocabulary acquisition in the modern Indonesian classroom context]]>
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