The denarius (Latin: [deːˈnaːriʊs]; pl.: dēnāriī, Latin: [deːˈnaːriiː]) was the standard Roman silver coin from its introduction in the Second Punic War c. 211 BC to the reign of Gordian III (AD 238–244), when it was gradually replaced by the antoninianus. It continued to be minted in very small quantities, likely for ceremonial purposes, until and through the Tetrarchy (293–313).

The word dēnārius is derived from the Latin dēnī "containing ten", as its value was originally of 10 assēs. The word for "money" descends from it in Italian (denaro), Slovene (denar), Portuguese (dinheiro), and Spanish (dinero). Its name also survives in the dinar currency.

Its symbol is represented in Unicode as 𐆖 (U+10196), a numeral monogram that appeared on the obverse in the Republican period, denoting the 10 asses ("X") to 1 denarius ("I") conversion rate. However it can also be represented as X̶ (capital letter X with combining long stroke overlay).

A predecessor of the denarius was first struck in 269 or 268 BC, five years before the First Punic War, with an average weight of 6.81 grams, or 1⁄48 of a Roman pound. Contact with the Greeks had prompted a need for silver coinage in addition to the bronze currency that the Romans were using at that time. This predecessor of the denarius was a Greek-styled silver coin of didrachm weight, which was struck in Neapolis and other Greek cities in southern Italy. These coins were inscribed with a legend that indicated that they were struck for Rome, but in style they closely resembled their Greek counterparts. They were rarely seen at Rome, to judge from finds and hoards, and were probably used either to buy supplies or to pay soldiers.

The first distinctively Roman silver coin appeared around 226 BC. Classical historians have sometimes called these coins "heavy denarii", but they are classified by modern numismatists as quadrigati, a term which survives in one or two ancient texts and is derived from the quadriga, or four-horse chariot, on the reverse. This, with a two-horse chariot or biga which was used as a reverse type for some early denarii, was the prototype for the most common designs used on Roman silver coins for a number of years.

Rome overhauled its coinage shortly before 211 BC, and introduced the denarius alongside a short-lived denomination called the victoriatus. The denarius contained an average 4.5 grams, or 1⁄72 of a Roman pound, of silver, and was at first tariffed at ten asses, hence its name, which means 'tenner'. It formed the backbone of Roman currency throughout the Roman Republic and the early Empire.

The denarius began to undergo slow debasement toward the end of the republican period. Under the rule of Augustus (27 BC – 14 AD) its weight fell to 3.9 grams (a theoretical weight of 1⁄84 of a Roman pound). It remained at nearly this weight until the time of Nero (AD 37–68), when it was reduced to 1⁄96 of a pound, or 3.4 grams. Debasement of the coin's silver content continued after Nero. Later Roman emperors also reduced its weight to 3 grams around the late 3rd century.

The value at its introduction was 10 asses, giving the denarius its name, which translates as "containing ten". In about 141 BC, it was re-tariffed at 16 asses, to reflect the decrease in weight of the as. The denarius continued to be the main coin of the Roman Empire until it was replaced by the antoninianus in the early 3rd century AD. The coin was last issued, in bronze, under Aurelian between 270 and 275 AD, and in the first years of the reign of Diocletian.