Three (5∶1∶3, 1∶1∶1, and 2∶1∶6) ternary phases were discovered in the K2MoO4−AMoO4−Zr(MoO4)2 system, where A is Mg or Mn. For A=Mg, we have synthesized 5∶1∶3 single crystals and determined their crystal structure from X-ray diffraction data (a CAD-4 automatic diffractometer, MoKα radiation, 1166 F(hkl), and R=0.026). The compound crystallizes in the trigonal system with space group R3c, a=10.576(1), c=37.511(3), A, Z=6, dcalc=3.576, and dmsd=3.54 g/cm3. The structure is a three-dimensional composite framework of alternating Mo tetrahedra and (Mg, Zr) octahedra, which are linked via the common O vertices. Potassium atoms of three kinds are located in large framework cavities. Their polyhedra (ten-vertex polyhedra and a cubeoctahedron) are linked together by common faces and edges to form infinite zigzag columns of a large section. When solving the structure, we refined the composition of the crystals and the distribution of Mg2+ and Zr4+ cations in the M(1) and M(2) positions resulting in the formula above.