(R)-4-(N,N-Dimethylammonio)-3-hydroxybutanoate [(R)-norcarnitine] monohydrate, C6H13NO3.H2O, Mr = 165.2, triclinic, P1, a = 5.9081 (5), b = 6.0438 (4), c = 6.9084 (7) A, alpha = 65.584 (6), beta = 81.957 (8), gamma = 77.771 (6) degrees, V = 219.1 (1) A3, Z = 1, Dx = 1.252 g cm-3, lambda(Cu K alpha) = 1.54184 A, mu = 8.46 cm-1, F(000) = 90, T = 296 K, R = 0.035 for 1763 observations (of 1772 unique data). Intermolecular hydrogen bonding dominates the structure. The molecule of water contacts three different zwitterions, with O...O distances 2.7767 (13), 2.720 (2) and 2.722 (2) A. A hydrogen bond between carboxylate and dimethylammonio links the zwitterions in head-to-tail motif, N...O 2.672 (2) A. Comparison with two other 4-ammonio-3-hydroxybutanoates reveals that the title compound adopts a different conformation along the backbone, N(+)--C--C(OH)--C--CO2-, with N(+)--C--C(OH)--C anti and C--C(OH)--C--CO2- gauche-.