In cardiomyocytes, calcium plays important roles as a signal in cardiac hypertrophy and contraction‐relaxation cycling. Elevation of Ca²⁺ concentration in myoplasm is associated with the onset and progression of hypertrophy as well as the enhancement of contractility. The cardiac Ca²⁺ ATPase (SERCA2a) of the sarcoplasmic reticulum plays a dominant role in lowering cytoplasmic calcium levels during relaxation and is regulated by phospholamban (PLN). To examine whether the modulation of SERCA2a activity results in the attenuation of cardiac hypertrophy and enhancement of contractility, we generated transgenic mice (TG) overexpressing a high calcium affinity SERCA2a mutant (K397/400E), lacking a functional association with PLN. In the TG hearts, the apparent affinity of SERCA2a for Ca²⁺ significantly increased compared with their nontransgenic littermate controls. The TG showed increased contraction and relaxation, with increases in the amplitude of Ca²⁺ transient and rapid Ca²⁺ decay. Upon induction of pressure overload by transverse aortic constriction, the TG developed less cardiac hypertrophy than littermate controls did. The activation of Ca²⁺‐sensitive protein kinase C by pressure overload was significantly attenuated in the TG hearts. Our findings indicate an association of SERCA2a activity with cardiac hypertrophy and thus a new therapeutic target for the prevention and treatment of cardiac hypertrophy.