Lablab purpureus (L.) Sweet (L. purpureus) has been reported to alleviate diarrhea, although the precise mechanism remains unclear. This study identified the primary active components of L. purpureus utilizing ultra-performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS). A rhubarb-induced diarrhea model in rats was utilized to assess the therapeutic efficacy of L. purpureus. Alterations in gut microbiota and fecal metabolism were analyzed via 16S rDNA analysis and targeted metabolomics. Flora elimination and fecal transplantation techniques were employed to deepen understanding of the role of intestinal flora in L. purpureus treatment. The study findings indicated that the main constituents of L. purpureus included trigonelline, piperidinic acid, and L-(–)-malic acid, among others. L. purpureus treatment significantly alleviated all diarrhea symptoms in rats, encompassing reduced fecal water content, weight loss, shortened colon length, diminished histological damage, and decreased inflammatory factors. Furthermore, L. purpureus significantly enhanced the expression of tight junction markers and restored the dysregulated intestinal flora in diarrheic rats by increasing Prevotella and reducing Lactobacillus. Additionally, the production of propionic acid and other short-chain fatty acids (SCFAs) increased in diarrheic rats treated with L. purpureus, suggesting a substantial alteration in the intestinal environment. Crucially, the protective efficacy of L. purpureus diminishes in the absence of gut flora. Subsequent fecal transplantation tests demonstrated that feces from the L. purpureus-treated group alleviated rhubarb-induced diarrhea, emphasizing the pivotal role of gut microbiota in the antidiarrheal efficacy of L. purpureus. In conclusion, our findings elucidate the underlying mechanisms of L. purpureus’ antidiarrheal action and its beneficial impact on intestinal microflora. Moreover, these results provide compelling evidence supporting the therapeutic use of L. purpureus for the treatment of diarrhea and its associated complications.