Endothelial dysfunction often accompanies sepsis. We aimed to explore the role of PCSK9 in septic endothelial dysfunction. Sepsis was induced by lipopolysaccharide (LPS) treatment of human umbilical vein endothelial cells (HUVECs) in vitro and cecal ligation and puncture (CLP) surgery in mice in vivo. Evolocumab (EVC) and Pep 2–8, PCSK9 inhibitors, were subsequently used to determine the role of PCSK9 in sepsis-induced endothelial dysfunction in vitro and in vivo, respectively. In addition, the TLR4 agonist, Kdo2-Lipid A ammonium (KLA), was used to determine the related mechanism. Protein expression of eNOS, VE-cadherin, PCSK9, TLR4, MyD88, p-p65, p65, NLRP3, ASC, and caspase-1 p20 in mice aortas and HUVECs was measured by western blotting, while mRNA expression of TNFα, IL-1β, and IL-18 was determined by qRT-PCR. The level of inflammatory cytokines of mouse aortas was visualized by immunohistochemistry. Vasodilation of the aorta was detected by vascular reactivity experiments. The 96-h survival rate after CLP was assessed. Our findings showed that the expression of eNOS and VE-cadherin decreased, and PCSK9 expression increased, in septic HUVECs or mice. Inhibition of PCSK9 increased eNOS and VE-cadherin expression. Activation of the TLR4/MyD88/NF-κB and NLRP3 pathways may be responsible for PCSK9-induced endothelial dysfunction in sepsis. Vascular reactivity tests and survival studies showed that inhibition of PCSK9 could prevent the decrease in endothelium-dependent vasodilation function and improve the survival rates of septic mice. In summary, our results suggested that increased PCSK9 expression during sepsis activated the TLR4/MyD88/NF-κB and NLRP3 pathways to induce inflammation, which resulted in vascular endothelial dysfunction and decreased survival rates. Thus, inhibition of PCSK9 may be a potential clinical therapeutic target to improve vascular endothelial function in sepsis.