AIMS: Low-density lipoprotein (LDL)-cholesterol is causally involved in atherosclerotic cardiovascular disease (ASCVD) pathogenesis. Pharmacological activation of the intracellular NAD + -dependent deacetylase Sirtuin-1 (SIRT1) reduces plasma LDL-cholesterol levels by increasing hepatic LDL-receptor (LDLR) expression, which intriguingly associates with atheroprotective effects. Recent studies have identified the presence of SIRT1 in plasma, however, its effects remain elusive. We found that plasma levels of SIRT1 to be decreased in atherosclerotic mice compared with wild-type controls and aimed to investigate the therapeutic potential of systemic SIRT1 restoration on lipid metabolism and plaque burden in atherosclerotic mice and dissect the underlying molecular mechanisms involved. METHODS AND RESULTS: Twelve-week-old apolipoprotein E-deficient (ApoE-/-) mice fed a high-cholesterol diet (1.25% w/w) were randomized to receive recombinant murine SIRT1(rmSIRT1) (n = 6; 0.3 mg/kg BW i.p.) or vehicle (n = 6; PBS) every third day over 4 weeks. Boosting systemic SIRT1 levels increased hepatic LDLR protein expression, reduced plasma LDL-cholesterol levels and decreased plaque progression in ApoE-/- mice. Yet, rmSIRT1 treatment did not change hepatic proprotein convertase subtilisin/kexin type 9 (PCSK9) expression but notably increased its deacetylated levels. Mechanistically, rmSirt1 directly bound to hepatic PCSK9 thereby promoting PCSK9 deacetylation involving 3 sites, namely Lys243, Lys421, and Lys506, as shown by mass spectrometric analyses. In vitro mutagenesis to triple deacetylation mimetic (3KR) reduced SIRT1-induced PCSK9 activity, as evidenced by increased cellular binding and association of 125I-LDL to hepatic LDLR. Finally, plasma levels of SIRT1 and PCSK9 were assessed at baseline in patients with acute coronary syndromes. In these patients, plasma SIRT1 levels correlated inversely with PCSK9 with high SIRT1 levels conferring a reduced risk of major adverse cardiovascular events (MACE). CONCLUSION: SIRT1 directly binds hepatic PCSK9 and decreases its activity by deacetylation, thereby enhancing LDL-cholesterol clearance by hepatic LDLR upregulation. Boosting circulating SIRT1 exerts atheroprotective effects in mice, with high levels associating with improved prognosis in patients with established ASCVD.