Ni[CH3(CH2)17PO3]·H2O was prepared and characterized by several techniques and the magnetic properties were measured by using a SQUID magnetometer. Preliminary refinement of the X-ray diffraction powder data by structure-less Le Bail fitting could be obtained and the compound was found to crystallise in the orthorhombic space group Pmn21 with a=5.478(7) Å, b=42.31(4) Å, c=4.725(3) Å. Ni(II)octadecyl phosphonate is lamellar and the structure consists of alternating inorganic and organic layers. The inorganic layers are interspersed by by-layers of the octadecyl substituent and van der Waals contacts are established between them. IR spectroscopy revealed all-trans configuration of the hydrocarbonic chain. A tilt angle of 48.2° between the chain axis and the (ac) plane could be estimated. The temperature dependence of the molar susceptibility plotted as 1/χ vs. T is linear above 100 K and it follows the Curie–Weiss law. The Curie, C, constant suggests the presence of Ni(II) ion in the S=1 spin state and the negative Weiss, θ, constant is indicative of antiferromagnetic nearest neighbour exchange interactions. Zero-field and field-cooled χ vs. T plots were then recorded. The plots show no overlap below 20 K, thus indicating that the compound is in an ordered magnetic state. The critical temperature has been located at the onset of the χ vs. T plot and was found to be TN=21 K. The magnetization vs. field plots, measured at different temperatures, provide the indication that the compound is a weak-ferromagnet below TN.