Rod-shaped single crystals of Sr4Pt10In21 were prepared from the elements in glassy-carbon crucibles in a high-frequency furnace. The structure of Sr4Pt10In21 was refined from single-crystal X-ray diffractometer data: C2/m, Ho4Ni10Ga21 type, a = 2322.62(7), b = 450.27(2), c = 1958.09(7) pm, β = 133.191(3)°, wR = 0.0464, 3200 F2 values and 107 variables. The three-dimensional [Pt10In21]δ− polyanionic network is stabilized through substantial Pt–In (269–313 pm Pt–In) and In–In (294–362 pm In–In) bonding. All platinum atoms have slightly distorted tri-capped trigonal prismatic coordination and the two crystallographically independent strontium atoms are located in penta-capped pentagonal prisms.