Mitochondrial dysfunction is a key factor in exacerbating pressure overload-induced cardiac hypertrophy and is linked to increased morbidity and mortality. ECSIT, a crucial adaptor for inflammation and mitochondrial function, has been reported to express multiple transcripts in various species and tissues, leading to distinct protein isoforms with diverse subcellular localizations and functions. However, whether an unknown ECSIT isoform exists in cardiac cells and its potential role in regulating mitochondrial function and pathological cardiac hypertrophy has remained unclear. This study identified a 42-kDa ECSIT isoform encoded by the transcript variant Ecsit-X4, which is highly expressed in the mitochondria of adult cardiomyocytes but down-regulated in hypertrophic human heart samples and TAC-treated mouse hearts. AAV9-mediated Ecsit-X4 gene therapy, administered either before or after TAC surgery, significantly attenuated cardiac hypertrophy. Cardiomyocyte-specific Ecsit deficiency worsened TAC-induced cardiac hypertrophy, while Ecsit-X4 compensation independently rescued hypertrophic phenotypes in Ecsit