Sodium selenite is the inorganic compound with the formula Na₂SeO₃. This salt is a colourless solid. The pink coloured pentahydrate Na₂SeO₃·5H₂O is the most common water-soluble selenium compound.

Sodium selenite usually is prepared by the reaction of selenium dioxide with sodium hydroxide:

The hydrate is stable in moist air, and converts to the anhydrous salt upon heating to 40 °C (104 °F) or storage in moist air.

According to X-ray crystallography, both anhydrous Na₂SeO₃ and its pentahydrate feature pyramidal SeO2−3. The Se−O distances range from 1.67 to 1.72 Å. Oxidation of this anion gives sodium selenate, Na₂SeO₄.

Together with the related barium and zinc selenites, sodium selenite is mainly used in the manufacture of colorless glass. The pink color imparted by these selenites cancels out the green color imparted by iron impurities.

Because selenium is an essential element, sodium selenite is an ingredient in dietary supplements such as multi-vitamin/mineral products, but supplements that provide only selenium use L-selenomethionine or a selenium-enriched yeast.

The US Food and Drug Administration approved a selenium supplement to animal diets; the most common form is sodium selenite for pet foods. According to one article:

"not much was known about which selenium compounds to approve for use in animal feeds when the decisions were made back in the 1970s .. At the time the regulatory action was taken, only the inorganic selenium salts (sodium selenite and sodium selenate) were available at a cost permitting their use in animal feed."