Metallic impurities in the silicon wafer bulk are one of the major efficiency-limiting factors in silicon solar cells. Gettering can be used to significantly lower the metal concentrations. Although gettering by silicon nitride films has been reported in literature, much remains unknown about its gettering behaviors and mechanisms. In this study, the gettering kinetics and mechanisms of silicon nitride films, from both plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD), are investigated. By monitoring the kinetics of iron loss from the silicon wafer bulk, it is confirmed that silicon nitride gettering takes place mainly via segregation, even at a low annealing temperature of 400 °C. Simulation of the gettering kinetics suggests the presence of an interfacial diffusion barrier in some cases, which slows down the transport of iron impurities from the silicon wafer bulk to the silicon nitride gettering regions. The activation energy of the segregation gettering process is estimated to be 0.9 ± 0.1 eV for the investigated PECVD silicon nitride film at 400–900 °C and 1.6 ± 0.5 eV for the investigated LPCVD silicon nitride film at 400–700 °C.