Berberine (BBR) is a non-prescription drug to treat various bacteria-associated diarrheas. However, BBR has also been reported to cause diarrhea in clinic, with underlying mechanisms poorly understood. Because altered gut microbial ecology is a potential basis for diarrhea, this study was conducted to investigate the impact of BBR on gut microbiota of treatment-emergent diarrhea. BBR treatment (200 mg/kg, i.g.) in normal rats exhibited no significant changes in serum biochemical parameters but mild diarrhea occurred, accompanied with the decreased gastrointestinal transit time and increased fecal moisture, suggestive of the local effects of BBR in the intestine. Colon histology revealed the decreased abundance of mucus-filled goblet cells in BBR group. Although BBR-treated rats had the enlarged cecum with watery caecal digesta, short-chain fatty acids concentration was significantly lower than control group. Additionally, BBR caused gut microbiota dysbiosis by evaluating the decreased observed species number and Shannon index. BBR increased the relative abundances of families Porphyromonadaceae and Prevotellaceae as well as genera Parabacteroides, Prevotellaceae_UCG-001 and Prevotellaceae_NK3B31_group. Spearman’s correlation analysis revealed family Prevotellaceae and genus Prevotellaceae_UCG-001 as the most prominent drivers of the BBR treatment-emergent diarrhea, correlating positively with fecal moisture but negatively with gastrointestinal transit time. This study therefore demonstrated that the treatment-emergent mild diarrhea of BBR was most likely due to the dysbiosis of the gut microbiota.