Single-phase NiCo₂O₄ (NCO) nanoparticles (NPs) with an average particle size of 12 (±3.5) nm were successfully synthesized as aggregates in urchin-like nanofibers via a hydrothermal route. Magnetization data measured as functions of temperature and magnetic field suggest a superparamagnetic-like behavior at room temperature, a ferrimagnetic transition around a Curie temperature T_C ∼ 200 K, and a spin blocking transition at a blocking temperature T_B ∼ 90 K, as observed at a field of 100 Oe. The spin blocking nature has been investigated by analyses of the field-dependence of T_B in the static magnetization and its frequency-dependence in the ac susceptibility data measured in zero-field cooling regime, both indicate a low-temperature spin glass-like state. Below T_B, the coercivity increases monotonically up to 1.7 kOe with decreasing temperature down to 5 K. Our results indicate that the magnetic behavior of NCO NPs, which is mainly determined by the cations' ratio, oxidation states, and site-occupancy, can be controlled by a synthesis in appropriate particle size and morphology.