Epithelial cells possess apical-basolateral polarity and form tight junctions (TJs) at the apical-lateral border, separating apical and basolateral membrane domains. The PAR3-aPKC-PAR6 complex plays a central role in TJ formation and apical domain development during tissue morphogenesis [1–4]. Inactivation and overactivation of aPKC kinase activity disrupts membrane polarity [5–8]. The mechanism that suppresses active aPKC is unknown. KIBRA, an upstream regulator of the Hippo pathway, regulates tissue size in Drosophila [9–11] and can bind to aPKC [12, 13]. However, the relationship between KIBRA and the PAR3-aPKC-PAR6 complex remains unknown. We report that KIBRA binds to the PAR3-aPKC-PAR6 complex and localizes at TJs and apical domains in epithelial tissues and cells. The knockdown of KIBRA causes expansion of the apical domain in MDCK three-dimensional cysts and suppresses the formation of apical-containing vacuoles through enhanced de novo apical exocytosis. These phenotypes are restored by inhibition of aPKC. In addition, KIBRA directly inhibits the kinase activity of aPKC in vitro. These results strongly support the notion that KIBRA regulates epithelial cell polarity by suppressing apical exocytosis through direct inhibition of aPKC kinase activity in the PAR3-aPKC-PAR6 complex.