This phase was discovered following direct fusion of the elements in welded Nb tubes at 550 degreesC and equilibration at 300 degreesC for 1 week. Single-crystal X-ray diffraction analysis reveals that KNa3In9 crystallizes in an orthorhombic system (Cmca, Z = 8, a = 9.960(1) Angstrom, b =16.564(2) Angstrom, c = 17.530(2) Angstrom, 23 degreesC). The structure contains a three-dimensional indium network built of layers of empty In-12 icosahedra that are each 12-bonded and interconnected by 4-bonded indium atoms that also form zigzag chains. All cations bridge between cluster faces or edges, and their mixed sizes appear critical to the stability of this particular structure, which does not occur in either binary system. Both empirical electron counting and EHTB band structure calculations on the macroanion indicate that the bonding in this structure is closed-shell, whereas resistivity and magnetic susceptibility measures show that the compound is a moderately poor metal.