Epithelial-mesenchymal transition (EMT) is a process by which epithelial-derived cells experience phenotypic transformation, gain mesenchymal-like cell phenotype under the condition of various physical and chemical factors. Many signaling molecules participate in regulating EMT during the process of embryo development, organ regeneration and tumor development. As the downstream effector of Hippo signal pathway, the role of YAP in EMT has been widely studied. It plays a dominant role in gene expression to control the cell proliferation and apoptosis, and organ development and regeneration. In addition, increasing evidences demonstrate that activity of YAP is closely related to tumor migration and invasion, along with the occurrence of EMT. Here, we reviewed the roles of YAP in EMT in tissue development, organ fibrosis and tumor development, and the progress of related molecular mechanisms, which is expected to broaden a new prospective for EMT studies and provide new molecular targets and diagnostic strategies for the treatment of related diseases.