The magnetic behavior of the reduced Nd 1.8 Sr 0.2 NiO 3.72 nickelate has been investigated by means of neutron-powder diffraction, magnetic susceptibility, and isothermal magnetization measurements. The most noticeable result is that, in spite of the large concentration of oxygen vacancies, both the Ni and Nd sublattices become antiferromagnetically ordered below T N 1 ≊320 K and T N 2 ≊13.5 K, respectively. Moreover, two spin reorientations of the Ni magnetic moments have been observed, one around T 1 ≊130 K and the other simultaneously to the cooperative ordering of the Nd sublattice. The existence of a weak ferromagnetic component below 70 K has also been clearly established from the analysis of the macroscopic magnetic measurements. Finally, a set of effective exchange parameters have been derived, and their values compared with those reported for stoichiometric Nd 2 NiO 4 .