Magnetic correlation in the quantum S = 1/2 square-lattice system (CuBr)Sr₂Nb₃O₁₀ has been studied by neutron diffraction. A novel commensurate in-plane, helical antiferromagnetic (AFM) ordering, characterized by the propagation vector k = (0 3/8 1/2), has been confirmed from the appearance of magnetic Bragg peaks below T_N ~ 7.5 K. The ordered moment at 2 K is found to be 0.79(7) μ_B/Cu²⁺-ion. The observed helical AFM structure differs from the ground state predicted theoretically from the J₁-J₂ model as well as from experimentally reported states for other quantum S = 1/2 square-lattice systems. However, the observed helical magnetic structure can be described in a J₁-J₂-J₃ model. Under a 4.5 Tmagnetic field, the spin-order changes drastically and is characterized by the propagation vector k₁ = (0 1/3 0.446) and a probable k₂ = (0 0 0) vector.