Mixed sodium nickel hydroxide phosphate, Na₂Ni₃(OH)₂(PO₄)₂, has been synthesized hydrothermally from the system NiCO₃–Na₄P₂O₇–NaCl–H₂O. Its monoclinic crystal structure has been determined by single crystal X-ray diffraction: a = 14.259(5), b = 5.695(2), c = 4.933(1) Å, β = 104.28(3)°, space group C2/m, Z = 2, ρ_c = 3.816 g cm⁻³, R = 0.026. The underlying spin model has been characterized in terms of first-principles electronic structure calculations. The compound is formed by alternating layers of [NiO₆] octahedra and [NaO₇] polyhedra, combined in the [100] direction with tetrahedral [PO₄] oxocomplexes and hydrogen bonds. The novel phase is treated as an isostructural variant of the two-dimensional potassium manganese hydroxide vanadate, K₂Mn₃(OH)₂(VO₄)₂, which can be formally obtained by morphotropic substitutions of all positions in the cationic sublattice. The stripe arrangement of Ni²⁺ ions (S = 1) within [NiO₄(OH)₂] layers of Na₂Ni₃(OH)₂(PO₄)₂ is unique in the sense that its magnetic topology places it in between widely discussed honeycomb and kagomé lattices. The Na₂Ni₃(OH)₂(PO₄)₂ is a low-dimensional magnet, which reaches the short-range correlation regime at T_max = 38.4 K and orders antiferromagnetically at T_N = 33.4 K.