We have been able, via a new synthetic route, to obtain a complete crystal structure of the title compound, tetra­aqua­barium hydro­xide iodide, [Ba(OH)I(H₂O)₄], for which the heavy atoms only were characterized by Kellersohn, ­Beckenkamp & Lutz [Z. Naturforsch. Teil B (1991), 46, 1279-1286]. In the present results, the H-atom positions could be located using X-ray data collection at low temperature. A three-dimensional network is built up via hydrogen bonds. It was also observed that the title compound undergoes hy­drolysis and might therefore be regarded as an intermediate in the formation of sol-gels, starting from BaI2 and leading to [Ba(OH)₂(H₂O)_x].