<jats:title>Abstract</jats:title><jats:p>Higher probability of the development of Crohn’s disease (CD) and ulcerative colitis (UC) as a possible consequence of the north-south gradient has been recently suggested. Living far north or south of the equator is manifested in fluctuation of vitamin D (vitD) levels depending on the season in both healthy and affected individuals. In the present study we investigate the possible link between the seasonal serum vitD level to the microbial composition of the lower gut of Inflammatory Bowel disease (IBD) patients using 16S rRNA sequencing. Decrease of serum vitD level in winter/spring season in a cohort of 35 UC patients and 39 CD patients was confirmed. Low gut microbiota composition of patients with IBD correlated with the serum level of 25(OH)D that directly coupled to seasonal variability of the sunshine in the central European countries. It is supposed to be related to increased abundance of <jats:italic>Actinobacteria</jats:italic> and <jats:italic>Proteobacteria</jats:italic> in UC and <jats:italic>Actinobacteria</jats:italic>, <jats:italic>Fusobacteria</jats:italic>, <jats:italic>Firmicutes</jats:italic> and <jats:italic>Bacteroidetes</jats:italic> in CD. In summer/autumn period, we observed a reduction in abundance of bacterial genera typical for inflammation like <jats:italic>Eggerthella lenta</jats:italic>, <jats:italic>Fusobacterium</jats:italic> spp., <jats:italic>Bacteroides</jats:italic> spp., <jats:italic>Collinsella aerofaciens</jats:italic>, <jats:italic>Helicobacter</jats:italic> spp., <jats:italic>Rhodococcus</jats:italic> spp., <jats:italic>Faecalibacterium prausnitzii</jats:italic>; and increased abundance of <jats:italic>Pediococcus</jats:italic> spp. and <jats:italic>Clostridium</jats:italic> spp. and of <jats:italic>Escherichia</jats:italic>/<jats:italic>Shigella</jats:italic> spp.</jats:p>