ETHNOPHARMACOLOGICAL RELEVANCE: Grape seed proanthocyanidin extract (GSPE), derived from Vitis vinifera L., has been used in traditional European and Mediterranean medicine to treat inflammation, vascular diseases, and wounds. Historically employed for "detoxification" and "hard lumps" (possibly tumors), its effects are now attributed to GSPE's antioxidant and antiproliferative properties. Modern studies suggest these mechanisms may underlie its potential anticancer applications, including against bladder cancer. AIM OF THE STUDY: To validate the traditional antitumor claims of grape seeds by investigating grape seed proanthocyanidin extract (GSPE)'s efficacy against bladder cancer (BCa) in vitro and in vivo, with a focus on its metabolic regulatory mechanisms. METHODS: We employed an integrated multi-omics approach combining in vitro assays (including MTT, colony formation, migration, and cell cycle/apoptosis analyses), in vivo orthotopic BCa models, and untargeted metabolomics using ultrahigh-performance liquid chromatography-Q Exactive™ mass spectrometry (UHPLC-Q-Exactive MS). Bioinformatics analysis was performed to examine inosine 5'-monophosphate dehydrogenase 1/2 (IMPDH1/2) expression in BCa tissues and evaluate their prognostic significance. Subsequently, IMPDH1 and IMPDH2 expression levels were quantified by qPCR and Western blot, and their enzymatic activity was assessed in vitro. Molecular docking was then performed to predict potential interactions between GSPE and IMPDH1/2. RESULTS: GSPE exhibited potent dose-dependent anti-tumor effects in BCa cells, suppressing proliferation, colony formation, and migration through induction of G0/G1 arrest and apoptotic cell death. In orthotopic BCa mouse models, GSPE administration achieved 82.66% tumor growth inhibition (p < 0.001) without inducing systemic toxicity or bladder irritation, as confirmed by normal serum biochemistry and histopathological analysis. Untargeted metabolomic profiling identified 307 significantly altered metabolites, with purine/pyrimidine metabolism showing the most profound dysregulation. Bioinformatics analysis revealed that elevated IMPDH1/2 expression in BCa tissues correlated with poor patient prognosis. Mechanistically, GSPE both dose-dependently downregulated IMPDH1/2 expression (at mRNA and protein levels), directly inhibited their enzymatic activity, and bound to IMPDH1/2 active sites with high affinity. CONCLUSION: Our findings demonstrate that GSPE serves as a novel IMPDH1/2-targeted therapeutic agent capable of disrupting nucleotide metabolism in BCa through dual inhibition of both enzymatic function and gene expression. While the exact active form(s) in vivo require further pharmacokinetic investigation, the potent anti-tumor efficacy and exceptional safety profile of intravesical GSPE support its significant clinical potential as a phytotherapeutic agent for BCa treatment.