Barium (Ba) is a chemical element with atomic number 56. It belongs to group 2 in the periodic table, the so called alkaline earth metals.

Barium is very chemically reactive and therefore not found in its free form in nature. There are several stable and naturally occurring barium isotopes, but the most abundant one is ¹³⁸Ba, with 56 protons and 82 neutrons.

The compound barium sulfate (BaSO₄) is sometimes used in medicine where it can be swallowed and then viewed in X-ray imaging to analyze, and diagnose problems with, the digestive system.