In the title hydrate, C(18)H(20)BrO(4)P·H(2)O, a staggered conformation is found when the organic mol-ecule is viewed down the central P-C bond, with the oxo and hydroxyl groups being diagonally opposite; each of the central P and C atoms has an S-configuration. The crystal structure features supra-molecular double chains along the b axis mediated by O(hydrox-yl)-H⋯O(oxo), O(water)-H⋯O(oxo), and O(water)-H⋯O(water) hydrogen bonds.