In adherens junctions, α-catenin links the cadherin–β-catenin complex to the actin-based cytoskeleton. α-catenin is a homodimer in solution, but forms a 1:1 heterodimer with β-catenin. The crystal structure of the α-catenin dimerization domain, residues 82–279, shows that α-catenin dimerizes through formation of a four-helix bundle in which two antiparallel helices are contributed by each protomer. A slightly larger fragment, comprising residues 57–264, binds to β-catenin. A chimera consisting of the α-catenin-binding region of β-catenin linked to the amino terminus of α-catenin 57–264 behaves as a monomer in solution, as expected, since β-catenin binding disrupts the α-catenin dimer. The crystal structure of this chimera reveals the interaction between α- and β-catenin, and provides a basis for understanding adherens junction assembly.