The Drosophila Yorkie (Yki) protein and its mammalian homolog Yes-associated protein (YAP) are potent growth promoters, and YAP overexpression is associated with multiple types of cancer. Yki and YAP are transcriptional coactivators and function as downstream effectors of the Hippo tumor suppressor pathway. The regulation of Yki and YAP by the Hippo signaling pathway has been extensively investigated; however, how they regulate gene expression is poorly understood. To identify additional regulators of Yki activity, we performed a genome-wide RNAi screen in Drosophila S2 cells. In this screen, we identified the conserved protein Mask (Multiple ankyrin repeats single KH domain) as a novel promoter of Yki activity in vitro and validated this function in vivo in Drosophila. We found that Mask is required downstream of the Hippo pathway for Yki to induce target-gene expression and that Mask forms complexes with Yki. The human Mask homolog MASK1 complexes with YAP and is required for the full activity of YAP. Additionally, elevated MASK1 expression is associated with worsened outcomes for breast cancer patients. We conclude that Mask is a novel cofactor for Yki/YAP required for optimal Yki/YAP activity during development and oncogenesis.