Osmium (Os) is a chemical element with atomic number 76. With a density of 22.53 g/cm³, it is the densest of all stable elements. It is also extremely rare, making up around 0.000000005% of the Earth's crust, or 50 ppt (parts per trillion).

There are several naturally occurring osmium isotopes, but ¹⁹²Os (76 protons, 116 neutrons) is the most abundant one.

Osmium was originally used in light bulbs (the German light bulb maker Osram's name is a combination of osmium and wolfram) but tungsten replaced osmium since it is both cheaper and more stable.