Although protozoa play a crucial role in ruminal microbial networks, their contribution to rumen fermentation and methane emissions remains controversial. Tannins, polyphenolic compounds derived from various plants, can affect microbial activity, fermentation processes, protein degradation, and methane production in the rumen. This study investigated the impact of dietary supplementation with chestnut, quebracho, and seaweed tannins on rumen fermentation parameters, protozoan populations, and enteric methane production. Rumen fluid (RF) samples were collected from four cannulated Simmental cows assigned to four dietary treatments: a control diet (0TAN) and three tannin-enriched diets: 150 g/day (150TAN), 200 g/day (200TAN), and 250 g/day (250TAN) in a 4 × 4 Latin-square design. The basal diet contained 64.3% meadow hay, 13.1% grass silage, 9% maize silage, 11.5% cereal concentrate, and 0.7% mineral concentrate. Most rumen fermentation parameters, including pH, total volatile fatty acids, acetate, propionate, acetate:propionate ratio, and isovalerate, were significantly influenced (P < 0.005) by the tannin-enriched diet, except for ammonia and butyrate concentrations. Tannin supplementation at 200 g/day, the concentrations of acetate, isovalerate, and the acetate ratio significantly increased (P < 0.001), whereas propionate production decreased. Significant effects (P < 0.001) were also observed with respect to the total number of protozoa and the relative abundance of protozoan genera. These findings suggest that dietary tannins modulate protozoan composition and the rumen microbial community, demonstrating antimethanogenic potential. The results confirm that tannins suppress rumen methanogenesis by reducing the size of protozoan population and enteric methane production.