In the mol-ecular structure of the title compound, also named penciclovir monohydrate, C(10)H(15)N(5)O(3)·H(2)O, the 4-hydr-oxy-3-hydroxy-methyl-but-1-yl group is connected to guanine through an N atom of the imidazole ring. Water mol-ecules stabilize the mol-ecular packing by forming O-H⋯O hydrogen bonds. A three-dimensional network is generated via inter-molecular N-H⋯N, N-H⋯O, O-H⋯N and O-H⋯O hydrogen bonding.