Tetrataenite (L1(0)-FeNi) is a promising candidate for use as a permanent magnet free of rare-earth elements because of its favorable properties. In this study, single-phase L1(0)-FeNi powder with a high degree of order was synthesized through a new method, nitrogen insertion and topotactic extraction (NITE). In the method, FeNiN, which has the same ordered arrangement as L1(0)-FeNi, is formed by nitriding A1-FeNi powder with ammonia gas. Subsequently, FeNiN is denitrided by topotactic reaction to derive single-phase L1(0)-FeNi with an order parameter of 0.71. The transformation of disordered-phase FeNi into the L1(0) phase increased the coercive force from 14.5 kA/m to 142 kA/m. The proposed method not only significantly accelerates the development of magnets using L1(0)-FeNi but also offers a new synthesis route to obtain ordered alloys in non-equilibrium states.