Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally found in its native form as elemental crystals. Tellurium is far more common in the universe as a whole than on Earth. Its extreme rarity in the Earth's crust, comparable to that of platinum, is due partly to its formation of a volatile hydride that caused tellurium to be lost to space as a gas during the hot nebular formation of Earth.

Tellurium-bearing compounds were first discovered in 1782 in a gold mine in Kleinschlatten, Transylvania (now Zlatna, Romania) by Austrian mineralogist Franz-Joseph Müller von Reichenstein, although it was Martin Heinrich Klaproth who named the new element in 1798 after the Latin tellus 'earth'. Gold telluride minerals are the most notable natural gold compounds. However, they are not a commercially significant source of tellurium itself, which is normally extracted as a by-product of copper and lead production.

Commercially, the primary use of tellurium is CdTe solar panels and thermoelectric devices. A more traditional application in copper (tellurium copper) and steel alloys, where tellurium improves machinability, also consumes a considerable portion of tellurium production.

Tellurium has no biological function, although fungi can use it in place of sulfur and selenium in amino acids such as tellurocysteine and telluromethionine. In humans, tellurium is partly metabolized into dimethyl telluride, (CH₃)₂Te, a gas with a garlic-like odor exhaled in the breath of victims of tellurium exposure or poisoning.

Tellurium has two allotropes, crystalline and amorphous. When crystalline, tellurium is silvery-white with a metallic luster. The crystals are trigonal and chiral (space group 152 or 154 depending on the chirality), like the gray form of selenium. It is a brittle and easily pulverized metalloid. Amorphous tellurium is a black-brown powder prepared by precipitating it from a solution of tellurous acid or telluric acid (Te(OH)₆). Tellurium is a semiconductor that shows greater electrical conductivity in certain directions depending on atomic alignment; the conductivity increases slightly when exposed to light (photoconductivity). When molten, tellurium is corrosive to copper, iron, and stainless steel. Of the chalcogens (oxygen-family elements), tellurium has the highest melting and boiling points, at 722.66 and 1,261 K (449.51 and 987.85 °C), respectively.

Crystalline tellurium consists of parallel helical chains of Te atoms, with three atoms per turn. This gray material resists oxidation by air and is not volatile.

Naturally occurring tellurium has eight isotopes. Six of those isotopes, ¹²⁰Te, ¹²²Te, ¹²³Te, ¹²⁴Te, ¹²⁵Te, and ¹²⁶Te, are stable. The other two, ¹²⁸Te and ¹³⁰Te, are slightly radioactive, with extremely long half-lives, including 2.2 × 10²⁴ years for ¹²⁸Te. This is the longest known half-life among all radionuclides and is about 160 trillion (10¹²) times the age of the known universe. Electron capture decay should occur for ¹²³Te, but is still unobserved.

A further 31 artificial radioisotopes of tellurium are known, with atomic masses ranging from 104 to 142 and with half-lives up to 19.31 days for ¹²¹Te. Also, 17 nuclear isomers are known, with half-lives up to 164.7 days for the same isotope. Except for beryllium-8 and beta-delayed alpha emission branches in some lighter nuclides, tellurium (¹⁰⁴Te to ¹⁰⁹Te) is the lightest element with isotopes known to undergo alpha decay.