The objective of this study was to determine how Saccharomyces cerevisiae supplementation affects rumen microbial diversity and function in Pogasi (Peranakan Ongole Grati Hasil Seleksi) beef cattle. The treatment included a diet supplemented with 2 g of yeast per animal daily compared to a control unsupplemented diet, using a 2 × 2 Latin square design. Venn diagram analysis identified 536 core microbial genera common for all groups, while unique genera in yeast-supplemented groups indicated an effect on microbial composition and diversity. Yeast addition increased microbial richness, as shown by rarefaction curves, and improved species evenness, suggesting a more stable ecosystem. Taxonomic analysis revealed Firmicutes and Bacteroidota as the dominant phyla, with yeast supplementation inducing minor changes in microbial composition. At the class level, Methanobacteria, Clostridia, and Bacteroidia were predominant, with a slight reduction in Methanobacteria in yeast-supplemented groups, suggesting a potential inhibitory effect on methanogenesis. Functional analysis showed alterations in metabolic pathways related to fibre degradation, volatile fatty acid production, and nitrogen metabolism. These findings indicate that yeast supplementation modifies microbial structure, stabilises the rumen microbiome, and improves fermentation efficiency, potentially reducing methane emissions. This study demonstrates that yeast supplementation is a promising strategy to improve rumen function and sustainability in beef cattle production.