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Dysprosium is a chemical element; it has symbol Dy and atomic number 66. It is a rare earth element in the lanthanide series with a metallic silver luster. Dysprosium is never found in nature as a free element, though, like other lanthanides, it is found in various minerals, such as xenotime. Naturally occurring dysprosium is composed of seven isotopes, the most abundant of which is 164Dy. Dysprosium was first identified in 1886 by Paul Émile Lecoq de Boisbaudran, but it was not isolated in pure form until the development of ion exchange techniques in the 1950s. Dysprosium has relatively few applications where it cannot be replaced by other chemical elements. It is used for its high thermal neutron absorption cross section in making control rods in nuclear reactors, for its high magnetic susceptibility in data storage applications, and as a component of Terfenol D (a magnetostrictive material). Soluble dysprosium salts are mildly toxic, while the insoluble salts are considered non toxic.

Dysprosium halides, such as DyF3 and DyBr3, tend to take on a yellow color. Dysprosium oxide, also known as dysprosia, is a white powder that is highly magnetic, more so than iron oxide. Two of the most abundant dysprosium carbonates, Dy2(CO3)3·2–3H2O (similar to the mineral tengerite (Y)), and DyCO3(OH) (similar to minerals kozoite (La) and kozoite (Nd), are known to form via a poorly ordered (amorphous) precursor phase with a formula of Dy2(CO3)3·4H2O. This amorphous precursor consists of highly hydrated spherical nanoparticles of 10–20 nm diameter that are exceptionally stable under dry treatment at ambient and high temperatures. Dysprosium forms several intermetallics, including the dysprosium stannides.

Naturally occurring dysprosium is composed of seven isotopes: 156Dy, 158Dy, 160Dy, 161Dy, 162Dy, 163Dy, and 164Dy. These are all considered stable, although only the last two are theoretically stable: the others can theoretically undergo alpha decay. Of the naturally occurring isotopes, 164Dy is the most abundant at 28%, followed by 162Dy at 26%. The least abundant is 156Dy at 0.06%. Dysprosium is the heaviest element to have isotopes that are predicted to be stable rather than observationally stable isotopes that are predicted to be radioactive. Twenty nine radioisotopes have been synthesized, ranging in atomic mass from 138 to 173. The most stable of these is 154Dy, with a half life of approximately 3 years, followed by 159Dy with a half life of 144.4 days. The least stable is 138Dy, with a half life of 200 ms. As a general rule, isotopes that are lighter than the stable isotopes tend to decay primarily by β+ decay, while those that are heavier tend to decay by β− decay. However, 154Dy decays primarily by alpha decay, and 152Dy and 159Dy decay primarily by electron capture. Dysprosium also has at least 11 metastable isomers, ranging in atomic mass from 140 to 165. The most stable of these is 165mDy, which has a half life of 1.257 minutes. 149Dy has two metastable isomers, the second of which, 149m2Dy, has a half life of 28 ns.

In 1878, erbium ores were found to contain the oxides of holmium and thulium. French chemist Paul Émile Lecoq de Boisbaudran, while working with holmium oxide, separated dysprosium oxide from it in Paris in 1886. His procedure for isolating the dysprosium involved dissolving dysprosium oxide in acid, then adding ammonia to precipitate the hydroxide. He was only able to isolate dysprosium from its oxide after more than 30 attempts at his procedure. On succeeding, he named the element dysprosium from the Greek dysprositos (δυσπρόσιτος), meaning "hard to get". The element was not isolated in relatively pure form until after the development of ion exchange techniques by Frank Spedding at Iowa State University in the early 1950s. Due to its role in permanent magnets used for wind turbines, it has been argued that dysprosium will be one of the main objects of geopolitical competition in a world running on renewable energy. But this perspective has been criticised for failing to recognise that most wind turbines do not use permanent magnets and for underestimating the power of economic incentives for expanded production. In 2021, Dy was turned into a 2 dimensional supersolid quantum gas.

While dysprosium is never encountered as a free element, it is found in many minerals, including xenotime, fergusonite, gadolinite, euxenite, polycrase, blomstrandine, monazite and bastnäsite, often with erbium and holmium or other rare earth elements. No dysprosium dominant mineral (that is, with dysprosium prevailing over other rare earths in the composition) has yet been found. In the high yttrium version of these, dysprosium happens to be the most abundant of the heavy lanthanides, comprising up to 7–8% of the concentrate (as compared to about 65% for yttrium). The concentration of Dy in the Earth's crust is about 5.2 mg/kg and in sea water 0.9 ng/L.

Dysprosium is used, in conjunction with vanadium and other elements, in making laser materials and commercial lighting. Because of dysprosium's high thermal neutron absorption cross section, dysprosium oxide–nickel cermets are used in neutron absorbing control rods in nuclear reactors. Dysprosium–cadmium chalcogenides are sources of infrared radiation, which is useful for studying chemical reactions. Dysprosium is increasingly in demand for the permanent magnets used in electric car motors and wind turbine generators. Neodymium–iron–boron magnets can have up to 6% of the neodymium substituted by dysprosium to raise the coercivity for demanding applications, such as drive motors for electric vehicles and generators for wind turbines. This substitution would require up to 100 grams of dysprosium per electric car produced. Based on Toyota's projected 2 million units per year, the use of dysprosium in applications such as this would quickly exhaust its available supply. The dysprosium substitution may also be useful in other applications because it improves the corrosion resistance of the magnets. Dysprosium is one of the components of Terfenol D, along with iron and terbium. Terfenol D has the highest room temperature magnetostriction of any known material, which is employed in transducers, wide band mechanical resonators, and high precision liquid fuel injectors. Dysprosium is used in dosimeters for measuring ionizing radiation. Crystals of calcium sulfate or calcium fluoride are doped with dysprosium. When these crystals are exposed to radiation, the dysprosium atoms become excited and luminescent. The luminescence can be measured to determine the degree of exposure to which the dosimeter has been subjected. Additionally, dysprosium has been used to create a two dimensional supersolid in a laboratory environment. Supersolids are expected to exhibit unusual properties, including superfluidity. Dysprosium iodide and dysprosium bromide are used in high intensity metal halide lamps. These compounds dissociate near the hot center of the lamp, releasing isolated dysprosium atoms. The latter re emit light in the green and red part of the spectrum, thereby effectively producing bright light. Several paramagnetic crystal salts of dysprosium (dysprosium gallium garnet, DGG; dysprosium aluminium garnet, DAG; dysprosium iron garnet, DyIG) are used in adiabatic demagnetization refrigerators. The trivalent dysprosium ion (Dy3+) has been studied due to its downshifting luminescence properties. Dy doped yttrium aluminium garnet (Dy:YAG) excited in the ultraviolet region of the electromagnetic spectrum results in the emission of photons of longer wavelength in the visible region. This idea is the basis for a new generation of UV pumped white light emitting diodes. The stable isotopes of dysprosium have been laser cooled and confined in magneto optical traps for quantum physics experiments. The first Bose and Fermi quantum degenerate gases of an open shell lanthanide were created with dysprosium. Because dysprosium is highly magnetic—indeed it is the most magnetic fermionic element and nearly tied with terbium for most magnetic bosonic atom—such gases serve as the basis for quantum simulation with strongly dipolar atoms.

Like many powders, dysprosium powder may present an explosion hazard when mixed with air and when an ignition source is present. Thin foils of the substance can also be ignited by sparks or by static electricity. Dysprosium fires cannot be extinguished with water. It can react with water to produce flammable hydrogen gas. Dysprosium chloride fires can be extinguished with water. Dysprosium fluoride and dysprosium oxide are non flammable. Dysprosium nitrate, Dy(NO3)3, is a strong oxidizing agent and readily ignites on contact with organic substances. Soluble dysprosium salts, such as dysprosium chloride and dysprosium nitrate are mildly toxic when ingested. Based on the toxicity of dysprosium chloride to mice, it is estimated that the ingestion of 500 grams or more could be fatal to a human (c. f. lethal dose of 300 grams of common table salt for a 100 kilogram human). The insoluble salts are non toxic.