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Holmium is a chemical element; it has symbol Ho and atomic number 67. It is a rare earth element and the eleventh member of the lanthanide series. It is a relatively soft, silvery, fairly corrosion resistant and malleable metal. Like many other lanthanides, holmium is too reactive to be found in native form, as pure holmium slowly forms a yellowish oxide coating when exposed to air. When isolated, holmium is relatively stable in dry air at room temperature. However, it reacts with water and corrodes readily, and also burns in air when heated. In nature, holmium occurs together with the other rare earth metals (like thulium). It is a relatively rare lanthanide, making up 1.4 parts per million of the Earth's crust, an abundance similar to tungsten. Holmium was discovered through isolation by Swedish chemist Per Theodor Cleve. It was also independently discovered by Jacques Louis Soret and Marc Delafontaine, who together observed it spectroscopically in 1878. Its oxide was first isolated from rare earth ores by Cleve in 1878. The element's name comes from Holmia, the Latin name for the city of Stockholm. Like many other lanthanides, holmium is found in the minerals monazite and gadolinite and is usually commercially extracted from monazite using ion exchange techniques. Its compounds in nature and in nearly all of its laboratory chemistry are trivalently oxidized, containing Ho(III) ions. Trivalent holmium ions have fluorescent properties similar to many other rare earth ions (while yielding their own set of unique emission light lines), and thus are used in the same way as some other rare earths in certain laser and glass colorant applications. Holmium has the highest magnetic permeability and magnetic saturation of any element and is thus used for the pole pieces of the strongest static magnets. Because holmium strongly absorbs neutrons, it is also used as a burnable poison in nuclear reactors.

Holmium is the eleventh member of the lanthanide series. In the periodic table, it appears in period 6, between the lanthanides dysprosium to its left and erbium to its right, and above the actinide einsteinium.

As with many lanthanides, holmium is usually found in the +3 oxidation state, forming compounds such as holmium(III) fluoride (HoF3) and holmium(III) chloride (HoCl3). Holmium in solution is in the form of Ho3+ surrounded by nine molecules of water. Holmium dissolves in acids. However, holmium is also found to exist in the +2, +1 and 0 oxidation states.

The isotopes of holmium range from 140Ho to 175Ho. The primary decay mode before the most abundant stable isotope, 165Ho, is positron emission, and the primary mode after is beta minus decay. The primary decay products before 165Ho are terbium and dysprosium isotopes, and the primary products after are erbium isotopes. Of the 35 synthetic radioactive isotopes that are known, the most stable one is holmium 163 (163Ho), with a half life of 4570 years. All other radioisotopes have ground state half lives not greater than 1.117 days, with the longest, holmium 166 (166Ho) having a half life of 26.83 hours, and most have half lives under 3 hours. 166m1Ho has a half life of around 1200 years. The high excitation energy, resulting in a particularly rich spectrum of decay gamma rays produced when the metastable state de excites, makes this isotope useful as a means for calibrating gamma ray spectrometers.

Holmium(III) oxide is the only oxide of holmium. It changes its color depending on the lighting conditions. In daylight, it has a yellowish color. Under trichromatic light, it appears orange red, almost indistinguishable from the appearance of erbium oxide under the same lighting conditions. The color change is related to the sharp emission lines of trivalent holmium ions acting as red phosphors. Holmium(III) oxide appears pink under a cold cathode fluorescent lamp. Other chalcogenides are known for holmium. Holmium(III) sulfide has orange yellow crystals in the monoclinic crystal system, Under high pressure, holmium(III) sulfide can form in the cubic and orthorhombic crystal systems. It can be obtained by the reaction of holmium(III) oxide and hydrogen sulfide at Holmium(III) selenide is also known. It is antiferromagnetic below 6 K.

All four trihalides of holmium are known. Holmium(III) fluoride is a yellowish powder that can be produced by reacting holmium(III) oxide and ammonium fluoride, then crystallising it from the ammonium salt formed in solution. Holmium(III) chloride can be prepared in a similar way, with ammonium chloride instead of ammonium fluoride. It has the YCl3 layer structure in the solid state. These compounds, as well as holmium(III) bromide and holmium(III) iodide, can be obtained by the direct reaction of the elements:

Organoholmium compounds are very similar to those of the other lanthanides, as they all share an inability to undergo π backbonding. They are thus mostly restricted to the mostly ionic cyclopentadienides (isostructural with those of lanthanum) and the σ bonded simple alkyls and aryls, some of which may be polymeric.

Holmium (Holmia, Latin name for Stockholm) was discovered by the Swiss chemists Jacques Louis Soret and Marc Delafontaine in 1878 who noticed the aberrant spectrographic emission spectrum of the then unknown element (they called it "Element X"). The Swedish chemist Per Teodor Cleve also independently discovered the element while he was working on erbia earth (erbium oxide). He was the first to isolate the new element. Using the method developed by the Swedish chemist Carl Gustaf Mosander, Cleve first removed all of the known contaminants from erbia. The result of that effort was two new materials, one brown and one green. He named the brown substance holmia (after the Latin name for Cleve's home town, Stockholm) and the green one thulia. Holmia was later found to be the holmium oxide, and thulia was thulium oxide.

Like all the other rare earth elements, holmium is not naturally found as a free element. It occurs combined with other elements in gadolinite, monazite and other rare earth minerals. No holmium dominant mineral has yet been found. The main mining areas are China, United States, Brazil, India, Sri Lanka, and Australia with reserves of holmium estimated as 400,000 tonnes. Holmium makes up 1.3 parts per million of the Earth's crust by mass. Holmium makes up 1 part per million of the soils, 400 parts per quadrillion of seawater, and almost none of Earth's atmosphere, which is very rare for a lanthanide. It makes up 500 parts per trillion of the universe by mass. Holmium is commercially extracted by ion exchange from monazite sand (0.05% holmium), but is still difficult to separate from other rare earths. The element has been isolated through the reduction of its anhydrous chloride or fluoride with metallic calcium. Holmium is relatively inexpensive for a rare earth metal with the price about 1000 USD/kg.

Glass containing holmium oxide and holmium oxide solutions (usually in perchloric acid) have sharp optical absorption peaks in the spectral range 200 to 900 nm. They are therefore used as a calibration standard for optical spectrophotometers. The radioactive but long lived 166m1Ho is used in calibration of gamma ray spectrometers. Holmium is used to create the strongest artificially generated magnetic fields, when placed within high strength magnets as a magnetic pole piece (also called a magnetic flux concentrator). Holmium is also used in the manufacture of some permanent magnets. Holmium doped yttrium iron garnet (YIG) and yttrium lithium fluoride have applications in solid state lasers, and Ho YIG has applications in optical isolators and in microwave equipment (e. g., YIG spheres). Holmium lasers emit at 2.1 micrometres. They are used in medical, dental, and fiber optical applications. It is also being considered for usage in the enucleation of the prostate. Since holmium can absorb nuclear fission bred neutrons, it is used as a burnable poison to regulate nuclear reactors. and for glass, providing yellow orange coloring. In March 2017, IBM announced that they had developed a technique to store one bit of data on a single holmium atom set on a bed of magnesium oxide. With sufficient quantum and classical control techniques, holmium may be a good candidate to make quantum computers.

Holmium plays no biological role in humans, but its salts are able to stimulate metabolism. Humans typically consume about a milligram of holmium a year. Plants do not readily take up holmium from the soil. Some vegetables have had their holmium content measured, and it amounted to 100 parts per trillion. Holmium and its soluble salts are slightly toxic if ingested, but insoluble holmium salts are nontoxic. Metallic holmium in dust form presents a fire and explosion hazard. Large amounts of holmium salts can cause severe damage if inhaled, consumed orally, or injected. The biological effects of holmium over a long period of time are not known. Holmium has a low level of acute toxicity.