BackgroundGut bacteria play a crucial role in the metabolism of bile acids (BA). Whether an association exists between the fecal microbiota composition and circulating BA levels in humans is poorly understood. Here, we investigated the relationship between fecal microbiota diversity and composition with plasma levels of BA in young adults.MethodsFecal microbiota diversity/composition was analyzed with 16S rRNA sequencing in 80 young adults (74% women; 21.9 ± 2.2 years old). Plasma levels of BA were measured using liquid chromatography-tandem mass spectrometry. PERMANOVA and Spearman correlation analyses were used to investigate the association between fecal microbiota parameters and plasma levels of BA.ResultsFecal microbiota beta (P = 0.025) and alpha diversity indexes ofevenness(rho = 0.237, P = 0.033), Shannon(rho = 0.313, P = 0.004),andinverse Simpson(rho = 0.283, P = 0.010) were positively associated with plasma levels of the secondary BA glycolithocholic acid (GLCA). The relative abundance of genera belonging to theFirmicutesandBacteroidetesphyla was positively correlated with plasma levels of GLCA (all rho ≥ 0.225, P ≤ 0.049). However, the relative abundance of species fromFirmicutesandBacteroidetesphyla were negatively correlated with plasma levels of primary and secondary BA (all rho ≤ − 0.220, P ≤ 0.045), except for the relative abundance ofBacteroides vulgatus, Alistipes onderdonkii,andBacteroides xylanisolvensspecies (Bacteroidetesphylum) that were positively correlated with the plasma levels of GLCA.ConclusionsThe relative abundance of specific fecal bacteria species is associated with plasma levels of BA in young adults. However, further investigations are required to validate whether the composition of the gut microbiota can regulate the plasma concentrations of BA in humans.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11306-023-02016-8.