Single crystals of Cs2V4O9 were grown in a CsBr flux and two polytypes of Cs2V4O9 have been identified by both single-crystal and powder X-ray diffraction. Phase 1, α-Cs2V4O9, crystallizes in the tetragonal system, space group P42/m, with a = 5.727(1) Å, c = 15.052(3) Å, Z = 2, and its structure has been refined based on F2 to R1 = 0.0809, wR2 = 0.1015 using all 1450 independent reflections from a nearly perfectly twinned crystal. Phase 2, β-Cs2V4O9, also crystallizes in the tetragonal system, space group I4̄d , with a = 5.726(2) Å, c = 30.062(8) Å, Z = 4, and its structure has been refined to R1 = 0.0475, wR2 = 0.0805 using all 537 independent reflections. Both structures consist of VO5 square pyramids that are linked by edge- and corner-sharing to form two-dimensional arrays with the alkali metals lying between the layers. Rb2V4O9 is isostructural with α-Cs2V4O9, and its structure has been refined using powder neutron and X-ray diffraction data. Rietveld analysis using powder X-ray diffraction data has confirmed that the α and β phases coexist in the polycrystals of Cs2V4O9 with a ratio of α:β ≈ 55:45. Magnetic properties were examined in the temperature range from 5 to 700 K. Rb2V4O9 and Cs2V4O9 exhibit antiferromagnetic short-range correlations at high temperatures. Consistent with the crystal structures, the magnetic behavior can be explained based on a two-dimensional Heisenberg model and the inductive mechanism proposed previously for similar vanadium oxides.