FeNiN has recently been recognized as an appropriate intermediate compound for synthesizing FeNi alloys with an L10-type crystal structure, via nitrogen insertion and a topotactic extraction technique. These alloys are promising candidates for rare-earth-free permanent magnets. Although it is important to understand the physical properties of FeNiN to synthesize high-performance L10-FeNi, knowledge in this regard is lacking; it is only known that the crystal structure is tetragonal. In this study, the physical properties of high-quality FeNiN are investigated with respect to both structure and magnetism. FeNiN nanoparticles are obtained via an ammonia nitriding process at 300 °C for 50 h using A1-FeNi nanoparticles as precursors. Neutron powder diffraction (NPD) analysis indicates that the magnetic structure is C-type antiferromagnetic with ferromagnetically coupled Fe chains along the c axis, while Ni is nonmagnetic. Mössbauer spectroscopy measurements for both 57Fe and 61Ni are consistent with the conclusions of NPD analysis. The χ-T curve of FeNiN shows a cusp associated with the antiferromagnetic phase transition at TN ≈ 178 K, and the asymptotic Curie temperature (or Weiss temperature) is positive at θp ≈ 100 K, as determined by the χ-1-T plot. Both the magnetic structure and magnetic moment of Fe estimated by first-principles calculations are in accordance with the experimentally obtained ones, and the estimated transition temperature is close to the experimentally determined value. The calculation results concerning the exchange coupling between various Fe sites qualitatively explain the observed positive θp and antiferromagnetic long-range order.