Genes encoding subunits of SWI/SNF (BAF) chromatin-remodeling complexes are collectively mutated in ∼20% of all human cancers^,. Although ARID1A is the most frequent target of mutations, the mechanism by which its inactivation promotes tumorigenesis is unclear. Here we demonstrate that Arid1a functions as a tumor suppressor in the mouse colon, but not the small intestine, and that invasive ARID1A-deficient adenocarcinomas resemble human colorectal cancer (CRC). These tumors lack deregulation of APC/β-catenin signaling components, which are crucial gatekeepers in common forms of intestinal cancer. We find that ARID1A normally targets SWI/SNF complexes to enhancers, where they function in coordination with transcription factors to facilitate gene activation. ARID1B preserves SWI/SNF function in ARID1A-deficient cells, but defects in SWI/SNF targeting and control of enhancer activity cause extensive dysregulation of gene expression. These findings represent an advance in colon cancer modeling and implicate enhancer-mediated gene regulation as a principal tumor-suppressor function of ARID1A.