Aberrant Wnt signaling mediated by mutations affecting APC (adenomatous polyposis coli) or β-catenin initiates the majority of human colorectal cancers (CRC) and drives tumorigenesis through the activation of specific genes such as MYC. We report here a novel association whereby another oncogenic transcription factor, MYB/c-Myb, is necessary for intestinal adenoma development directed by activated Wnt signaling. APCMin/+ mice in which c-myb is haploinsufficient survive longer than wild-type APCMin/+ animals due to a delay in adenoma formation. Intestinal adenomas from APCMin/+ mice were assessed and found to have high levels of c-myc gene expression. We explored the relationship between activated Wnt signaling and MYB in regulating MYC and found activated β-catenin in combination with MYB induces robust upregulation of MYC promoter activity, as well as endogenous MYC mRNA and protein expression, in human cells. This cooperation occurred through independent binding of MYB and β-catenin to the MYC promoter. These data highlight a cooperative function for MYB in the context of activated Wnt signaling and provide a molecular basis for the expression of MYC in CRC.