IQ motif-containing GTPase-activating protein (IQGAP) scaffolding proteins regulate many essential cellular processes including growth factor receptor signaling, cytoskeletal rearrangement, adhesion, and proliferation and are highly expressed in many cancers. Using genetically engineered human skin tissue in vivo, we demonstrate that diminished, sub-physiologic expression of IQGAP1 or IQGAP3 is sufficient to maintain normal epidermal homeostasis, whereas significantly higher levels are required to support tumorigenesis. To target this tumor-specific IQGAP requirement in vivo, we engineered epidermal keratinocytes to express individual IQGAP protein domains designed to compete with endogenous IQGAPs for effector protein binding. Expression of the IQGAP1-IQ motif decoy domain in epidermal tissue in vivo inhibits oncogenic Ras-driven mitogen-activated protein kinase signaling and antagonizes tumorigenesis, without disrupting normal epidermal proliferation or differentiation. These findings define essential non-redundant roles for IQGAP1 and IQGAP3 in the epidermis and demonstrate the potential of IQGAP antagonism for cancer therapy.