Unicellular magnetic microorganisms include magnetotactic bacteria and some protist species. Although magnetosome magnetite in bacteria (prokaryotes) is well studied, little is known regarding the characteristics and origin of magnetic minerals in protists (eukaryotes). Stercomata stored within tests of the hadal foraminifera R. bilocularis from the Mariana Trench (6980-10,911 m depth), contain magnetite crystals. As a result, this species can orient in accordance with magnetic fields. The magnetite differs chemically and physically from that in the surrounding sediments. The crystals also differ from bacterial magnetosomes in being of variable size, porous structure, not arranged in chains, and encapsulated in a lipid membrane. Putting available evidence together indicates a biological origin of the magnetite, although a sedimentary source cannot be eliminated. This is the first record of a magnetic protist from hadal depths, opening a new window for the biomagnetism in the Earth's extreme environment.