Patients with ovarian cancer (OC) are at high risk of developing transcoelomic metastasis in the early stages, which is strongly associated with increased mortality rates. However, the mechanism by which OC cells disseminate from the primary site and colonize distant sites remains unknown. Here, through an in vivo genome-wide CRISPR/Cas9 screen, we identified NBL1, which increased dramatically in OC patients during peritoneal metastasis, as a key factor promoting the transcoelomic metastasis of OC. Overexpression of NBL1 in OC cells greatly promotes the transcoelomic metastasis. When OC cells disseminate into the peritoneal cavity, they induce the transition of peritoneal epithelial cells to mesothelial cells, ultimately activating the Jak/Stat3 signaling pathway. Thus, we show a NBL1-mediated crosstalk between peritoneum epithelial cells and mesothelial cells that supports a metastasis-promoting process.