Lanthanum ( /ˈlænθənəm/) is a chemical element with the symbol La and atomic number 57. Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and other rare earth elements. Lanthanum is a malleable, ductile, and soft metal that oxidizes rapidly when exposed to air. It is produced from the minerals monazite and bastnäsite using a complex multistage extraction process. Lanthanum compounds have numerous applications as catalysts, additives in glass, carbon lighting for studio lighting and projection, ignition elements in lighters and torches, electron cathodes, scintillators, and others. Lanthanum carbonate (La₂(CO₃)₃) was approved as a medication against renal failure.

Lanthanum is a soft, malleable, silvery white metal which has hexagonal crystal structure at room temperature. At 310 °C, lanthanum changes to a face-centered cubic structure, and at 865 °C into a body-centered cubic structure. Lanthanum easily is oxidized (a centimeter-sized sample will completely oxidize within a year) and is therefore used as in elemental form only for research purposes. For example, single La atoms have been isolated by implanting them into fullerene molecules. If carbon nanotubes are filled with those lanthanum-encapsulated fullerenes and annealed, metallic nanochains of lanthanum are produced inside carbon nanotubes.