Yellowcake (also known as urania and uranic oxide) is concentrated uranium oxide, obtained through the milling of uranium ore. It is usually represented by the formula U₃O₈. It is radioactive, forming a coarse powder which is insoluble in water and contain about 80% uranium oxide, and melts at approximately 2878°C. It is created by passing raw uranium ore through crushers and grinders to produce "pulped" ore. This is then bathed in sulfuric acid to leach out the uranium. Yellowcake is what remains after drying and filtering. The yellowcake produced by most modern mills is actually brown or black, not yellow; the name comes from the colour and texture of the concentrates produced by early mining operations.

Yellowcake is used in the preparation of fuel for nuclear reactors, where it is processed into purified uranium dioxide (UO₂) for use in fuel rods.

Yellowcake is unenriched uranium, and cannot by itself be used for a nuclear weapon. In a reactor, however, it may be used to breed plutonium, which can be used in weapons. It may also be enriched, by being converted to uranium hexafluoride gas (UF₆), for isotope separation by diffusion or centrifugal force to produce enriched uranium suitable for use in weapons and reactors.