By-products from agro-industry can represent a valuable resource for animal feeding to increase the sustainability of animal productions. Phenolic compounds, which can be found in several agro-industrial by-products, can modulate the activity of rumen microbiota and influence the concentration of fatty acids (FAs) associated with positive effects on human health such as vaccenic acid (VA, C18:1 t11). Hazelnut skin is a by-product that can be potentially used in ruminants’ diets due to its high content in phenolic compounds and crude fat. In a previous study, the dietary inclusion of hazelnut skin led to the enrichment of intramuscular fat with health-promoting FAs (i.e., VA and polyunsaturated FAs). Furthermore, a higher content of stearic acid (C18:0) and VA was observed in the rumen of the lambs fed the diet with the inclusion of hazelnut skin. However, the microbial communities in the rumen of the lambs fed the control diet versus the diet with the addition of hazelnut skin were not characterized (Priolo et al., 2021, Anim Feed Sci Technol. 272:114794). In this study, the microbial communities in the rumen of the lambs fed the two diets were characterized to gain more insight into the effects of the inclusion of hazelnut skin on rumen microbiota. Microbial DNA was extracted, and high-throughput sequencing of 16S rRNA gene amplicons was performed. Six taxa ([Eubacterium] nodatum group, Acidaminococcus, Dialister, Erysipelotrichaceae UCG-002, Megasphaera and Olsenella) were positively correlated with the concentration of C18:1 t10, a FA which can have negative effects both on human and animal health. Furthermore, their relative abundance was higher in the rumen of lambs fed the control diet. Our results suggest that dietary administration of hazelnut skin can decrease the relative abundance of microorganisms correlated to C18:1 t10 production in the rumen of animals fed a concentrate-based diet.