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Uranium-238, often written as 238U, is the most abundant isotope of natural uranium, accounting for about 99.3% of Uranium found in nature. It has a half-life of approximately 4.468 billion years and decays by alpha emission to thorium-234, initiating the uranium-238 decay chain that ultimately leads to the stable isotope lead-206.

In natural uranium, 238U dominates the isotopic composition, with trace amounts of 235U and 234U. Its long half-life makes it a useful clock for dating geological materials, particularly in conjunction with uranium-235 in radiometric dating methods. The 238U decay chain releases heat that contributes to the thermal evolution of the Earth's crust and mantle.

238U is not fissile with thermal neutrons; it is considered fertile because it can absorb a neutron to become 239U, which beta decays to neptunium-239 and then to plutonium-239. Pu-239 is fissile and can sustain a nuclear chain reaction, a principle exploited in breeder reactors to convert abundant 238U into usable fuel.

Applications of 238U include its role in radiometric dating (U-Pb dating), where the long half-life provides a reliable timescale for old rocks. It also contributes to the radiogenic heat production within the Earth. In addition to scientific uses, uranium ore processing and fuel cycles rely on managing 238U alongside lighter isotopes, with 238U comprising the bulk of natural uranium used in various industrial contexts.

Health and safety concerns arise from its radiotoxicity and chemical toxicity. When inhaled or ingested, uranium isotopes, including 238U, pose risks from alpha radiation and chemical exposure, requiring appropriate shielding and handling protocols.