GTP cyclohydrolase (GCH1) is the rate-limiting enzyme for tetrahydrobiopterin (BH4) biosynthesis. The catalysis of BH4 biosynthesis is tightly regulated for physiological neurotransmission, inflammation, and vascular tone. Paradoxically, BH4 has emerged as an oncometabolite regulating tumor growth, but the effects on tumor development remain controversial. Here we found that GCH1 potentiates the growth of triple-negative breast cancer (TNBC) and HER2+ breast cancer and transforms non-tumor breast epithelial cells. Independent of BH4 production, GCH1 protein induced epithelial-to-mesenchymal transition IEMT) by binding to Vim, which was mediated by HSP90. Conversely, GCH1 ablation impaired tumor growth, suppressed Vim in TNBC, and inhibited EGFR/ERK signaling while activating the p53 pathway in estrogen receptor-positive tumor cells. GCH1 deficiency increases tumor cell sensitivity to HSP90 inhibition and endocrine treatments. Additionally, high GCH1 corelated with poor breast cancer survival. Together, this study reveals an enzyme-independent oncogenic role of GCH1, presenting it as a potential target for therapeutic development.