Understanding ruminal microbiota and diet-host breed interactions under forage feeding conditions is essential for optimizing rumen fermentation and improving feed efficiency in small ruminants. This study aimed to investigate the effects of different ratios of condensed tannin-rich Sericea lespedeza (SL; Lespedeza cuneata) in the diets on changes and interactions of ruminal microbiota and host species (i.e., sheep and goats). Katahdin sheep (n = 12) and Alpine goats (n = 12) at approximately 10 to 12 mo of age were blocked by body weight (BW = 30.3 and 25.5 kg, respectively) and randomly assigned to one of the 3 treatments. Diets contained 75% coarsely ground forage and 25% concentrate. The forages were 1) 100% alfalfa hay (AL), 2) 100% SL, and 3) 50% AL + 50% SL (ASL). In the present study, the diversity and composition of ruminal microbiota differed between sheep and goats fed similar diets. Based on the taxonomic analysis, there was a distinct clustering pattern (P < 0.05) for sheep by diets, but such a pattern was not observed for goats (P > 0.1). The most predominant phyla were Firmicutes, Bacteroidetes, Ascomycota, and methanogen species of Methanobrevibactor sp. in the rumen of sheep and goats, regardless of diets. The Bacteroidetes and Ascomycota were enriched in sheep fed AL and ASL. In contrast, these microbial phyla were enhanced in goats fed tannin-rich SL diets, with the diet-by-host species interaction (P < 0.02) for the Bacteroidetes phylum. Sheep rumen fluid samples showed a higher degree of variability in microbial community composition compared to goat rumen fluid samples. The relative proportion of the Aspergillus fungi population was reduced to 90.7% in the SL group compared with the AL group, regardless of host species. The antimicrobial activity of tannins and greater sensitivities of selected microbiota species to these tannin compounds during SL feeding in sheep and goats perhaps caused this difference. The results from this study suggest that differences in the microbiota were associated with differences in diets and host species. Therefore, this study provides a better understanding of ruminal microbiota and diet-host species interactions under various tannin-rich diets, which could advance consolidative information on rumen microbiome community diversity changes and may improve sheep and goat production.