The sestertius, or sesterce, was an ancient Roman coin. During the Roman Republic it was a small, silver coin issued only on rare occasions. During the Roman Empire it was a large brass coin.

The name sestertius (originally semis-tertius) means "2 ½", the coin's original value in asses, and is a combination of semis "half" and tertius "third", that is, "the third half" (0 ½ being the first half and 1 ½ the second half) or "half the third" (two units plus half the third unit, or halfway between the second unit and the third). Parallel constructions exist in Danish with halvanden (1 ½), halvtredje (2 ½) and halvfjerde (3 ½). The form sesterce, derived from French, was once used in preference to the Latin form, but is now considered old-fashioned.

It is abbreviated as HS (originally IIS).

The sestertius was introduced ca. 211 BC as a small silver coin valued at one-quarter of a denarius (and thus one hundredth of an aureus). A silver denarius was supposed to weigh about 4.5 grams, valued at ten grams, with the silver sestertius valued at two and one-half grams. In practice, the coins were usually underweight.