Depleted uranium (DU; also referred to in the past as Q-metal, depletalloy or D-38) is uranium with a lower content of the fissile isotope ²³⁵
U
than natural uranium. Natural uranium contains about 0.72% ²³⁵
U
, while the DU used by the U.S. Department of Defense contains 0.3% ²³⁵
U
or less. The less radioactive and non-fissile ²³⁸
U
constitutes the main component of depleted uranium. Uses of DU take advantage of its very high density of 19.1 grams per cubic centimetre (0.69 lb/cu in) (68.4% denser than lead).

Civilian uses include counterweights in aircraft, radiation shielding in medical radiation therapy and industrial radiography equipment, and containers for transporting radioactive materials. Military uses include armor plating and armor-piercing projectiles.

Most depleted uranium arises as a by-product of the production of enriched uranium for use as fuel in nuclear reactors and in the manufacture of nuclear weapons. Enrichment processes generate uranium with a higher-than-natural concentration of lower-mass-number uranium isotopes (in particular ²³⁵
U
, which is the uranium isotope supporting the fission chain reaction) with the bulk of the feed ending up as depleted uranium, in some cases with mass fractions of ²³⁵
U
and ²³⁴
U
less than a third of those in natural uranium. Since ²³⁸
U
has a much longer half-life than the lighter isotopes, DU emits less alpha radiation than natural uranium. DU from nuclear reprocessing has different isotopic ratios from enrichment–by-product DU, from which it can be distinguished by the presence of ²³⁶
U
. The only known natural source of uranium with a ²³⁵
U content significantly different from 0.72% is found in the natural nuclear fission reactor at Oklo, Gabon. It can be "fingerprinted" as different in origin from manmade depleted uranium by the ²³⁴
U content, which is 55 ppm in uranium from the Oklo Mine as well as all other natural sources, but will be lower in depleted uranium in accordance with the degree of depletion.

DU is about 60% as radioactive as natural uranium. Most of the alpha radiation comes from ²³⁸
U
and ²³⁴
U
whereas beta radiation comes from ²³⁴
Th
and ²³⁴
Pa
that are formed within a few weeks.

The use of DU in munitions is controversial because of concerns about potential long-term health effects. Normal functioning of the kidney, brain, liver, heart, and numerous other systems can be affected by exposure to uranium, a toxic metal. It is only weakly radioactive because of the long radioactive half-life of ²³⁸
U
(4.468 × 10⁹ or 4,468,000,000 years) and the low amounts of ²³⁴
U
(half-life about 246,000 years) and ²³⁵
U
(half-life 700 million years). The biological half-life (the average time it takes for the human body to eliminate half the amount in the body) for uranium is about 15 days. The aerosol or spallation frangible powder produced by impact and combustion of depleted uranium munitions can potentially contaminate wide areas around the impact sites, leading to possible inhalation by human beings.

The actual level of acute and chronic toxicity of DU is also controversial. Several studies using cultured cells and laboratory rodents suggest the possibility of leukemogenic, genetic, reproductive, and neurological effects from chronic exposure. According to an article in Al Jazeera, DU from American artillery is suspected to be one of the major causes of an increase in the general mortality rate in Iraq since 1991. A 2005 epidemiology review concluded: "In aggregate the human epidemiological evidence is consistent with increased risk of birth defects in offspring of persons exposed to DU." A 2021 study concluded that DU from exploding munitions did not lead to Gulf War illness in American veterans deployed in the Gulf War. According to a 2013 study, despite the use of DU by coalition forces in Fallujah, no DU has been found in soil samples taken from the city, although another study of 2011 had indicated elevated levels of uranium in tissues of the city inhabitants.