Iron pyrite nanocubes with particle sizes of around 80–120 nm have been synthesized via a facile solvothermal method. Time-dependent characterization found that the iron(II) chloride and sulfur precursors first react to form sheet-like amorphous Fe_1−xS, which transforms into pyrite FeS₂ nanocubes upon further heating. As an anode material for lithium ion batteries, the as-synthesized pyrite FeS₂ nanocubes were found to deliver a reversible discharge capacity of 540 mA h g⁻¹ after cycling for 150 cycles at a current density of 1 A g⁻¹. Even at higher current density of 5 A g⁻¹, the pyrite FeS₂ electrode still managed to give a stable discharge capacity of about 220 mA h g⁻¹. The enhanced lithium storage properties are attributed to its higher specific surface area that can provide more lithium ion (Li⁺) reaction sites, leading to less polarization and better cycling performance.