X-ray powder diffraction data, unit-cell parameters, and space group for thiamphenicol, C12H15Cl2NO5S, are reported [a = 17.346(3), b = 15.341(0), c = 5.790 (2) Å, α = β = γ = 90°, unit-cell volume V = 1540.8(3) Å3, Z = 4, and space group P212121]. All measured lines were indexed and are consistent with the P212121 space group. No detectable impurities were observed.