Because endothelial nitric oxide synthase (eNOS) has anti-inflammatory and anti-arteriosclerotic functions, it has been recognized as one of the key molecules essential for the homeostatic control of blood vessels other than relaxation of vascular tone. Here, we examined whether telmisartan modulates eNOS function through its pleiotropic effect. Administration of telmisartan to mice significantly increased the phosphorylation level of eNOS (Ser1177) in the aortic endothelium, but administration of valsartan had no effect. Similarly, telmisartan treatment of human umbilical vein endothelial cells significantly increased the phosphorylation levels of AMP-activated protein kinase (Thr172) and eNOS and the concentration of intracellular guanosine 3′,5′-cyclic monophosphate (cGMP). Furthermore, pretreatment with a p38 mitogen-activated protein kinase (p38 MAPK) inhibitor suppressed the increased phosphorylation level of eNOS and intracellular cGMP concentration. These data show that telmisartan increases eNOS activity through Ser1177 phosphorylation in vascular endothelial cells mainly via p38 MAPK signaling.