The fabrication of Si₃N₄ ceramics typically requires high temperatures (above 1700 °C) and prolonged sintering time to achieve densification, resulting in high energy consumption and increased manufacturing costs. Moreover, reports on the fabrication of dense Si₃N₄ ceramics with good mechanical properties under MPa-level pressure and low temperatures are rare. In this work, we propose a low-temperature rapid spark plasma sintering strategy involving the introduction of fine-grained β-Si₃N₄ powder with high lattice strain energy as an “additive”. Dense biphasic Si₃N₄ ceramics, predominantly α-Si₃N₄, were successfully fabricated at a mechanical pressure of 200 MPa and a temperature of 1300 °C, achieving a relative density of 97%. The application of high pressure promoted particle rearrangement and uniform liquid‒phase distribution, providing additional driving forces for sintering. The introduction of β-Si₃N₄ seeds facilitated an in-situ solution–reprecipitation process, enabling rapid densification with a minimal liquid phase and without significant grain growth, resulting in nanometer-scale grains. The Si₃N₄ sample prepared at 1350 °C exhibited a desirable combination of high hardness (18.5 $\pm$0.3 GPa) and fracture toughness (6.7 $\pm$0.2 MPa·m^1/2). The results demonstrate that by adjusting the sintering temperature and time, the phase composition and mechanical properties of the ceramics can be flexibly tailored. This work holds significant potential for industrial manufacturing and provides valuable insights into low-temperature strategies for ceramic fabrication.