The aureus (pl. aurei — "golden") was a gold coin of ancient Rome valued at 25 silver denarii. The aureus was regularly issued from the 1st century BC to the beginning of the 4th century, when it was replaced by the solidus. The aureus is about the same size as the denarius, but is heavier due to the higher density of gold.

Before the time of Julius Caesar the aureus was struck very infrequently, usually to make large payments from captured booty. Caesar struck the coin more frequently and standardized the weight at Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{1}{40}

After the reign of Marcus Aurelius the production of aurei decreased, and the weight was further decreased to Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{1}{50}

The solidus was first introduced by Diocletian around 301 AD, struck at 60 to the Roman pound of pure gold (and thus weighing about 5.5 g each) and with an initial value equal to 1,000 denarii. However, Diocletian's solidus was struck only in small quantities, and thus had only minimal economic effect. The solidus was reintroduced by Constantine I in 312 AD, permanently replacing the aureus as the gold coin of the Roman Empire. The solidus was struck at a rate of 72 to a Roman pound of pure gold, each coin weighing twenty-four Greco-Roman carats, or about 4.5 grams of gold per coin. By this time, the solidus was worth 275,000 increasingly debased denarii.