The reaction of selenium and NbCl₅ with arsenic in the Lewis-acidic ionic liquid BMImCl·4.8AlCl₃ at 100°C yielded dark-red block-shaped crystals of Nb₂Se₄(AlCl ₄)₄, which immediately decompose when exposed to humid air. Arsenic takes the part of the reducing agent for niobium as well as selenium. The crystal structure was described in the monoclinic space group P2₁/n (no. 14) with a = 898.0(1) pm, b = 991.3(1) pm, c = 1629.1(2) pm, and β = 92.43(1)° at 296(1) K. The unit cell contains two C _2h symmetric Nb₂(Se₂)₂(AlCl ₄)₄ molecules. The latter consist of a central rectangular bipyramid [Nb₂(Se₂)₂]⁴⁺ and four η²-coordinating AlCl₄^- tetrahedra. Diamagentism of the compound and DFT-based real-space bonding analysis imply a chemical bond between the niobium(IV) cations, which are 292.2(2) pm apart. Nb₂Se₄(AlCl₄)₄ can be interpreted as the AlCl₃ adduct of NbSe₂Cl₂.