In the present study, we aimed to investigate the therapeutic mechanisms of carrot juice fermented with Lactobacillus rhamnosus GG (LGG) on type 2 diabetic mellitus (T2DM) based on the regulation of gut microbiota. Carrot juice fermented with LGG was enriched with free phenols, organic acids and short-chain fatty acids (SCFAs). Supplementation of carrot juice fermented with LGG (DFCL) could favorably regulate blood glucose, insulin, antioxidant capacity and morphology of the pancreas and kidney in the diabetic rats, accompanied by an increase of SCFAs in the cecum. Furthermore, high-throughput sequencing (HTS) analysis revealed that DFCL supplementation altered the composition of gut microbiota, showing increased relative abundances of functionally relevant enterotypes, such as Christensenellaceae_R-7_group, Oscillibacter, Ruminococcaceae_UCG-013, Lachnospiraceae_NK4A136_group and Akkermansia. In addition, Spearman's correlation analysis revealed that Desulfovibrio, Ruminococcaceae and Alloprevotella were closely correlated with biochemical biomarkers. Meanwhile, DFCL treatment regulated the expressions of genes involved in glucose metabolism at the mRNA and protein levels.