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The crystal structure of a new modification of NiSeO3 is described together with its magnetic properties. The new polymorph crystallizes in the monoclinic system, space group C 2/c, with unit cell parameters a = 15.4965(7) A, b = 9.8250(4) A, c = 14.7278(8) A, β = 110.610(4)° and Z = 8. The crystal structure was solved from single crystal data, R = 0.0252. The structure can be considered as being layered along the bc-plane with [NiO6] octahedra arranged into two sub-groups. The layers are further connected to adjacent layers via groups of two edge-sharing [NiO6] octahedra. The Ni octahedra are tilted and are either edge or corner sharing, giving rise to two different Ni–Ni interactions. The lone pairs of Se4+ occupy non-bonding regions of tunnels along [001] and [010]. The complexity of the crystal structure is also reflected in the complex low-temperature magnetic state. An antiferromagnetic-like transition was observed at Ttr = 14.4 K. A possible structural rearrangement closely above Ttr complicates the details of the low-temperature state. The antiferromagnetic-like transition disagrees with the features of a three-dimensional (3D) Neel antiferromagnetic ordering. This is mostly because of the fact that the assumed bulk easy-axis susceptibility at the lowest temperature remains at high paramagnetic value. The consequences of the complex structure on basic magnetic parameters, zero-field splitting energy D and exchange coupling J, and accordingly on the magnetic properties, are discussed.