Greigite (Fe3S4), a ferrimagnetic sulfide mineral with a spinel structure analogous to Fe3O4, was employed in magnetorheological (MR) fluids. Fe3S4 nanoparticles were synthesized via a solution process, with magnetic properties verified through vibrating sample magnetometry (VSM). The MR performance and suspension stability were assessed using a rotational rheometer and Turbiscan, and the results were compared to those of Fe3O4-based suspensions. When dispersed in silicone oil, the Fe3S4 suspension demonstrated MR performance that matched or surpassed that of Fe3O4-based suspensions. Despite the significant density mismatch between the particles and the liquid medium, the rough surface morphology of Fe3S4 nanoparticles enhances suspension stability by increasing interparticle friction and reducing sedimentation rates. Furthermore, Fe3S4’s natural abundance and low production cost provide economic advantages for MR fluid manufacturing, while its nontoxic nature makes it particularly suitable for biomedical applications.