The sestertius was an ancient Roman coin. During the Roman Republic it was a small silver coin and during the Roman Empire it was a large bronze coin. The name sestertius is a combination of semis, meaning half, and tres, meaning three, which when combined with an adjectival termination means three with the last equal half resulting in two and a half, the coin's original value in asses. In older English texts the French form sesterce is sometimes used.

The sestertius was introduced ca. 211 BC as a small silver coin that was one quarter of a denarius (and thus one hundredth of an aureus), and itself valued at ten asses. The sestertius had a theoretical weight of one scripulum (about 1.125 grams). When the denarius was retariffed to sixteen asses (due to the gradual reduction in the size of bronze denominations), the sestertius was accordingly revalued to four asses, still equal to one quarter of a denarius. It was produced sporadically, far less often than the denarius, through 44 BC.

In 23 BC, with the coinage reform of Augustus, the denomination of sestertius was introduced as the large bronze denomination. It and the dupondius were produced out of a golden-colored alloy known to numismatists as orichalcum. The sestertius was produced as the largest bronze denomination until the middle of the third century AD. During the end of its issue, when sestertii were reduced in size and quality, the double sestertius was issued first by Trajan Decius and later in large quantity by Postumus. The double sestertius was distinguished from the sestertius by the radiate crown worn by the emperor, a device used to distinguish the dupondius from the as and the antoninianus from the denarius.

Sestertius of Hadrian, dupondius of Antoninus Pius, and as of Marcus Aurelius

The sestertius was also used as a standard unit of account. Large values were recorded in terms of sestertia milia, thousands of sestertii, with the milia often omitted and implied.

Records from Pompeii show a slave being sold at auction for 6252 sesterces.

The sestertius is highly valued by numismatists, due to their large size they gave celators (engravers) a large area to produce detailed portraits and reverse types. Very high quality examples can sell for over a thousand dollars at auction as of 2004.

See also: Roman currency.