The H19 lncRNA, a highly abundant and conserved imprinted gene, has been implicated in many essential biological processes and diseases. However, the function of H19 in the heart remains unknown. In this study, we investigated the function and underlying mechanism of H19 in regulating cardiomyocyte hypertrophy. We first detected the expression of H19 and its encoded miR-675 in both normal and diseased hearts and verified their up-regulations in pathological cardiac hypertrophy and heart failure. Adenovirus-mediated expression and a siRNA-mediated silence of H19 showed that H19 overexpression reduced cell size both at baseline and in response to phenylephrine, whereas knock-down of H19 induced cardiomyocyte hypertrophy. Overexpression or knock-down of miR-675 in cardiomyocytes demonstrated that miR-675 also inhibited cardiomyocyte hypertrophy. Moreover, inhibition of miR-675 reversed the reduction of cardiomyocyte size in H19-overexpressing cardiomyocytes, while infection with an adenovirus carrying H19 fragment without pre-miR-675 (H19-Tru) or with mutant sequences of pre-miR-675 (H19-Mut) failed to reduce cardiomyocyte size, indicating that miR-675 mediated the inhibitory effect of H19 on cardiomyocyte hypertrophy. We also identified that CaMKII delta was a direct target of miR-675 and partially mediated the effect of H19 on cardiomyocyte hypertrophy. Furthermore, in vivo silencing of miR-675 using a specific antagomir in a pressure overload-induced mouse model of heart failure increased cardiac CaMKII delta expression and exacerbated cardiac hypertrophy. These findings reveal a novel function of H19-miR-675 axis targeting CaMKII delta as a negative regulator of cardiac hypertrophy, suggesting its potential therapeutic role in cardiac diseases.