ABSTRACT Aquaporin 9 (AQP9) expression is significantly elevated in vulnerable carotid plaque (VCP). Hence, we probed the mechanism of AQP9 in VCP formation. The VCP model was established in ApoE −/− C57BL/6 mice. Dataset GSE163154 was analyzed by R software. Human aortic endothelial cells (HAECs) were incubated with 50 µg/mL oxidized low‐density lipoprotein (ox‐LDL) and 20 mM l ‐(+)‐lactic acid for 24 h. Mice (AQP9 overexpression plasmid) and HAECs (AQP9 overexpression/dynamin‐related protein 1 [DRP1] silencing plasmids) were infected by lentivirus. Mouse plasma lipid level was estimated. The histopathological condition of model mice was observed by oil red lipid staining, hematoxylin–eosin (H&E) staining, and Masson staining. Levels of AQP9 and DRP1 in model mice and HAECs were quantified by quantitative real‐time polymerase chain reaction (qRT‐PCR). Levels of AQP9, DRP1, and mitochondrial fission‐/endothelium–mesenchymal transition (EndMT)‐related factors in model mice and HAECs were assayed by western blot. Lactate level in model mice was detected. Promoter histone lactylation level of DRP1 was measured by chromatin immunoprecipitation (ChIP). Behaviors of HAECs were tested by cell counting kit‐8 (CCK‐8), colony formation test, and scratch test. AQP9 was highly expressed in intraplaque hemorrhage patients. AQP9 overexpression promoted levels of DRP1, lactate, histone lactylation, mitochondrial fission factor, vimentin, and N ‐cadherin, while inhibiting vascular endothelial (VE)‐cadherin level and plaque stability in model mice and facilitating viability, proliferation, and migration of HAECs. DRP1 silencing reversed the impacts of AQP9 overexpression on cell viability, proliferation, migration, and levels of mitochondrial fission‐/EndMT‐related factors in HAECs. AQP9 enhances DRP1‐mediated mitochondrial fission by lactate and thus promotes EndMT to exacerbate the VCP formation.