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 TN ∼ 7.5 K. The ordered moment at 2 K is found to be 0.79(7) μB/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 T magnetic 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.