We investigated the dynamic behavior of ultrafine NiFe₂O₄ nanoparticles (average size D = 3.5 nm) that exhibit anomalous low temperature magnetic properties such as low saturation magnetization and high-field irreversibility in both M(H) and ZFC–FC processes. Besides the expected blocking of the superspin, observed at T₁≈45 K, the system undergoes a magnetic transition at T₂≈6 K. For the latter, frequency- and temperature-resolved dynamic susceptibility data reveal characteristics that are unambiguously related to collective spin freezing: the relative variation (per frequency decade) of the in-phase susceptibility peak temperature is ~0.025, critical dynamics analysis yields an exponent zν = 9.6 and a zero-field freezing temperature T_F = 5.8 K, and, in a magnetic field, T_F(H) is excellently described by the de Almeida–Thouless line . Moreover, out-of-phase susceptibility versus temperature datasets collected at different frequencies collapse on a universal dynamic scaling curve. All these observations indicate the existence of a spin-glass-like surface layer that surrounds the superparamagnetic core and undergoes a transition to a frozen state upon cooling below 5.8 K.