Epithelioid hemangioendothelioma, first described in early 1980s,is a rare malignant vascular neoplasm with significant morbidity and mortality. Approximately 50% of epithelioid hemangioendotheliomas exhibit intravascular endothelial growth, yet their cellular origin, pathogenesis, and effective treatment remain undefined. Here, we identified stable nuclear expression of endothelial YAP1 (Yes1-associated transcriptional regulator) in pathological tissue samples from patients with epithelioid hemangioendothelioma. Mice expressing stable nuclear form of YAP1 in endothelial cells recapitulated the human intravascular epithelioid hemangioendothelioma phenotype. Sustained YAP1 activity induced mitosis and aberrant expression of lymphatic and epithelioid genes in blood endothelial cells. These results show sustained activation of endothelial YAP1 as a causal mechanism for intravascular epithelioid hemangioendothelioma.