Californium is a synthetic chemical element; it has symbol Cf and atomic number 98. It was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory) by bombarding curium with alpha particles (helium-4 ions). It is an actinide element, the sixth transuranium element to be synthesized, and has the second-highest atomic mass of all elements that have been produced in amounts large enough to see with the naked eye (after einsteinium). It was named after the university and the U.S. state of California.

Two crystalline forms exist at normal pressure: one above and one below 900 °C (1,650 °F). A third form exists at high pressure. Californium slowly tarnishes in air at room temperature. Californium compounds are dominated by the +3 oxidation state. The most stable of californium's twenty known isotopes is californium-251, with a half-life of 898 years. This short half-life means the element is not found in significant quantities in the Earth's crust. ²⁵²Cf, with a half-life of about 2.645 years, is the most common isotope used and is produced at Oak Ridge National Laboratory (ORNL) in the United States and Research Institute of Atomic Reactors in Russia.

Californium is one of the few transuranium elements with practical uses. Most of these applications exploit the fact that certain isotopes of californium emit neutrons. For example, californium can be used to help start up nuclear reactors, and it is used as a source of neutrons when studying materials using neutron diffraction and neutron spectroscopy. It can also be used in nuclear synthesis of higher mass elements; oganesson (element 118) was synthesized by bombarding californium-249 atoms with calcium-48 ions. Users of californium must take into account radiological concerns and the element's ability to disrupt the formation of red blood cells by bioaccumulating in skeletal tissue.

Californium is a silvery-white actinide metal with a melting point of 900 ± 30 °C (1,650 ± 50 °F) and an estimated boiling point of 1,743 K (1,470 °C; 2,680 °F). The pure metal is malleable and is easily cut with a knife. Californium metal starts to vaporize above 300 °C (570 °F) when exposed to a vacuum. Below 51 K (−222 °C; −368 °F) californium metal is either ferromagnetic or ferrimagnetic (it acts like a magnet), between 48 and 66 K it is antiferromagnetic (an intermediate state), and above 160 K (−113 °C; −172 °F) it is paramagnetic (external magnetic fields can make it magnetic). It forms alloys with lanthanide metals but little is known about the resulting materials.

The element has two crystalline forms at standard atmospheric pressure: a double-hexagonal close-packed form dubbed alpha (α) and a face-centered cubic form designated beta (β). The α form exists below 600–800 °C with a density of 15.10 g/cm³ and the β form exists above 600–800 °C with a density of 8.74 g/cm³. At 48 GPa of pressure the β form changes into an orthorhombic crystal system due to delocalization of the atom's 5f electrons, which frees them to bond.

The bulk modulus of a material is a measure of its resistance to uniform pressure. Californium's bulk modulus is 50±5 GPa, which is similar to trivalent lanthanide metals but smaller than more familiar metals, such as aluminium (70 GPa).

Californium exhibits oxidation states of 4, 3, or 2. It typically forms eight or nine bonds to surrounding atoms or ions. Its chemical properties are predicted to be similar to other primarily 3+ valence actinide elements and the element dysprosium, which is the lanthanide above californium in the periodic table. Compounds in the +4 oxidation state are strong oxidizing agents and those in the +2 state are strong reducing agents.

The element slowly tarnishes in air at room temperature, with the rate increasing when moisture is added. Californium reacts when heated with hydrogen, nitrogen, or a chalcogen (oxygen family element); reactions with dry hydrogen and aqueous mineral acids are rapid.