Diosgenin [or (22R,25R)-spirost-5-en-3β-ol] is the starting material of the Marker degradation, a cheap semi-synthesis of progesterone, which has been designated as an Inter-national Historic Chemical Landmark. Thus far, a single X-ray structure for diosgenin is known, namely its dimethyl sulfoxide solvate [Zhang et al. (2005 ▶). Acta Cryst. E61, o2324-o2325]. We have now determined the structure of the hemihydrate, C(27)H(42)O(3)·0.5H(2)O. The asymmetric unit contains two diosgenin mol-ecules, with quite similar conformations, and one water mol-ecule. Hy-droxy groups in steroids and water mol-ecules form O-H⋯O hydrogen-bonded R(5) (4)(10) ring motifs. Fused edge-sharing R(10) rings form a backbone oriented along [100], which aggregates the diosgenin mol-ecules in the crystal structure.