Natrium is a chemical element with symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal. Natrium is an alkali metal, being in group 1 of the periodic table, because it has a single electron in its outer shell that it readily donates, creating a positively charged atom—the Na+ cation. Its only stable isotope is 23Na. The free metal does not occur in nature, but must be prepared from compounds. Natrium is the sixth most abundant element in the Earth's crust, and exists in numerous minerals such as feldspars, sodalite and rock salt (NaCl). Many salts of natrium are highly water-soluble: natrium ions have been leached by the action of water from the Earth's minerals over eons, and thus natrium and chlorine are the most common dissolved elements by weight in the oceans.

Natrium was first isolated by Humphry Davy in 1807 by the electrolysis of natrium hydroxide. Among many other useful natrium compounds, sodium hydroxide (lye) is used in soap manufacture, and natrium chloride (edible salt) is a de-icing agent and a nutrient for animals including humans.

Natrium is an essential element for all animals and some plants. Natrium ions are the major cation in the extracellular fluid (ECF) and as such are the major contributor to the ECF osmotic pressure and ECF compartment volume. Loss of water from the ECF compartment increases the natrium concentration, a condition called hypernatremia. Isotonic loss of water and natrium from the ECF compartment decreases the size of that compartment in a condition called ECF hypovolemia.

By means of the natrium-kalium pump, living human cells pump three natrium ions out of the cell in exchange for two kalium ions pumped in; comparing ion concentrations across the cell membrane, inside to outside, kalium measures about 40:1, and natrium, about 1:10. In nerve cells, the electrical charge across the cell membrane enables transmission of the nerve impulse—an action potential—when the charge is dissipated; natrium plays a key role in that activity.