FeS2 nanomaterials with two-dimensional features hold great promise for electrochemical and photovoltaic applications. However, the preparation of ultrathin FeS2 nanosheets is still challenging because of the lack of a tailor-made approach. In this work, FeS2 nanosheets with a thickness of 2.1 nm are prepared through a Fe3O4-seeded approach. Uniform Fe3O4 nanoparticles are foremost synthesized via the standard method in organic media. The injection of a S solution leads to the replacement of O in Fe3O4 through anion-exchange, which generates (110) facet-enriched FeS2 nuclei. The subsequent (110) facet-mediated oriented attachment and fusion of FeS2 nuclei produce ultrathin FeS2 nanosheets. As catalysts in the hydrogen evolution reaction, FeS2 nanosheets exhibit good electrochemical activity.