Abstract Reindeer digestion is impossible without the symbiotic microorganisms that live in its rumen. The composition of plant fibers present in the diet of reindeer is characterized by an increased content of plant fiber, hemicellulose, lignin, and secondary lichen metabolites. The Arctic is characterized by extremely scarce food resources even in summer. In winter the nutritional value of the diet is further reduced. The digestion of such feeds is provided only by the enzyme systems of microorganisms that live in the rumen of ruminants. Therefore, the purpose of our study was to determine the changes occurring in the composition of the reindeer’ rumen bacterial community inhabiting the Nenets region of the Russian Arctic. Rumen contents were collected from 20 reindeer in winter and summer 2017. To identify the bacterial community, 16S rRNA sequencing was used on the MiSeq (Illumina). Statistical processing of the results of NGS using the PERMANOVA analysis showed that the rumen bacterial communities in winter and summer have significant differences (P = 0.001). Taxonomic analysis using the GreenGenes database showed that, in general, at the phylum level in the community, representatives of Firmicutes (29.98–52.67%) and Bacteroidetes (33.55–51.87%) dominated. Significant differences were shown for microorganisms associated with the fermentation of plant polysaccharides. In winter, a significant increase in bacteria of the genera Succiniclasticum (P < 0.001), Paraprevotellaceae (P < 0.001), Coprococcus (P < 0.001), Butyrivibrio (P < 0.001), Prevotella (P < 0.001), Ruminococcus (P < 0.001). Thus, there is a succession of the bacterial community of the reindeer rumen associated with the change of seasons, affecting minor groups of microorganisms. In turn, it is associated with the availability of various components of the diet and the predominance of more or less easily digestible substances in it. The study was supported by a grant of Russian Science Foundation Project No. 17-76-20026.