Introduction: Septic cardiomyopathy remains a leading cause of death in critically ill patients. Myocardial edema and inflammatory cell recruitment to cardiac interstitial space are proposed mechanisms underlying sepsis-induced contractile dysfunction. Hypothesis: Evaluate the role of endothelial microRNA-155 (miR-155) in experimental septic cardiomyopathy. Methods: Experimental sepsis was induced using LPS injection in WT (n=20) and miR-155-/- (KO; n=20) male mice. Cardiac function was evaluated through echocardiography. Expression of NOS2, NOS3, VASP phosphorylation and myocardial nitrite/nitrate content was evaluated. Cardiac microvascular (MV) permeability was assessed with evans blue-albumin conjugate leaking to extravascular space and myocardial water content. Mouse cardiac MV endothelial cells (MCMVEC; CD31+Sca-1+CD45-) were isolated with FACS. Quantification of miR-155, cytokine mRNA profile and adhesion molecules (ICAM1, VCAM1 and E-selectin) was performed. Human cardiac MV endothelial cells (HCMVEC) were transfected with anti-miR-155 or scramble LNA probe and incubated with LPS (100ng/mL) or vehicle for 24h. Evaluation of cytokine mRNA profile, NOS isoforms, VASP phosphorylation and adhesion molecules was performed. Monocyte adhesion assay was performed using Dil-stained THP1 monocytic cells. Results: LPS exposure increased miR-155 expression in the myocardium, MCMVEC and HCMVEC. KO mice presented with attenuated LPS-induced cardiac dysfunction, MV permeability, NOS2 expression and VASP phosphorylation, compared with WT. KO MCMVEC presented with attenuated LPS-induced cytokine expression and adhesion molecules, compared with WT cells. Mir-155 inhibition in HCMVEC prevented LPS-induced overexpression of cytokines and adhesion molecules, nitric oxide production and monocyte adhesion, through SOCS1-mediated inhibition of STAT3 and NF-kB signaling pathways. Conclusions: Endothelial miR-155 is upregulated in experimental septic cardiomyopathy. MiR-155 knockout attenuates LPS-induced myocardial dysfunction, edema and MV permeability. In MCMVEC and HCMVEC, miR-155 inhibition blunted LPS-induced nitric oxide production, cytokine and cell adhesion molecules overexpression.