A polycrystalline sample of LaSr2Cr2SbO9 has been synthesised using a standard ceramic method and characterized by x-ray and neutron diffraction, magnetometry and electron microscopy. The perovskite-related compound crystallises in the triclinic space group I1̅ with unit cell parameters of a=5.5344(6) Å, b=5.5562(5) Å, c=7.8292(7) Å, α=89.986(12)°, β=90.350(5)° and γ=89.926(9)° at room temperature. The two crystallographically-distinct, six-coordinate cation sites are occupied by Cr3+ and Sb5+ in ratios of 0.868(2):0.132(2) and 0.462(2):0.538(2). Ac and dc magnetometry revealed that LaSr2Cr2SbO9 is ferrimagnetic below 150 K with a magnetisation of ~1.25 µB per formula unit in 50 kOe at 5 K. Neutron diffraction showed that the cations on the two sites order in a G-type arrangement with a mean Cr3+ moment of 2.17(1) µB at 5 K, consistent with a magnetisation of 1.32 µB per formula unit.