Wüstite, Fe_1‐xO, is an important component in the mineralogy of Earth's lower mantle and may also be a component of the core. Therefore its high pressure‐temperature behavior, including its electronic structure, is essential to understanding the nature and evolution of Earth's deep interior. We performed X‐ray diffraction and radiometric measurements on wüstite in a laser‐heated diamond anvil cell, finding an insulator‐metal transition at high pressures and temperatures. Our data show a negative slope for this apparently isostructural phase boundary, which is characterized by a volume decrease and emissivity increase. The metallic phase of FeO is stable at conditions of the lower mantle and core, which has implications for the highP‐Tcharacter of Fe‐O bonds, magnetic field propagation, and lower mantle conductivity.