Ovarian cancer (OC) peritoneal metastasis (OCPM) is a major cause of high mortality of OC, in which cancer cells incubated in ascites evolve various mechanisms to survive. Hippo/YAP singling plays multiple roles in carcinogenesis, however, its roles in OCPM have remained elusive. Here, we report that restriction of YWHAB-mediated YAP cytoplasmic retention is a critical mechanism underlying OCPM stemness maintenance. Combined tandem mass tag- and tissue microarray-based proteomic studies revealed YWHAB down-regulation in post-neoadjuvant chemotherapy OCPM tissues, which was confirmed in no-neoadjuvant-chemotherapy-response tissues, isolated OCPM stem cells, and induced cisplatin-resistant cells. Knockdown of YWHAB promoted stemness and resistance in parental complete or near-complete primary OCPM and OVCAR3 cells in vitro and in vivo. Mechanistic study showed that YWHAB directly bound to YAP and promoted YAP cytoplasmic retention and thus YWHAB restriction promoted YAP activity and stemness in OCPM in the cells in which the Hippo/YAP signaling was constitutively activated by overloaded constitutively active YAP (YAP5SA), and the effect of YWHAB knockdown was significantly abolished. The SH3 binding domain in YAP is critical for YWHAB-YAP binding. Alteration in the 5mc methylation level in the YWHAB promoter was observed in OCPM stem cells. In summary, our results reveal that restriction of YWHAB-mediated YAP cytoplasmic retention is a critical mechanism underlying OCPM stemness maintenance. Our findings suggest that YAP would be a therapeutic target for suppressing OCPM stemness caused by YWHAB restriction.