Capitolium, in Antiquity, a famous fort or castle, on the Mons Capitolinus at Rome, in which was a temple dedicated to Jupiter (thence also denominated Capitolinus), where the senate anciently assembled. It had its name of capitulum from caput, a man's head, said to have been found fresh, and yet bleeding, upon digging the foundation of the temple built in honour of Jupiter. Arnobius adds, that the man's name was Tolus, whence capitulum.

The first foundations of the capitol were laid by Tarquin the Elder, in the year of Rome 139. His successor Servius raised the walls, and Tarquin the Proud finished it in the year 221; but it was not consecrated till the third year after the expulsion of the kings, and the establishment of the consulate. The ceremony of the dedication of the temple was performed by the consul Horatius in 246.

The capitol consisted of three parts; a nave sacred to Jupiter, and two wings, the one consecrated to Juno, the other to Minerva. It was ascended by stairs. The frontispiece and sides were surrounded with galleries, in which those who were honoured with triumphs entertained the senate at a magnificent banquet, after the sacrifice had been offered to the gods. Both the inside and outside were enriched with an infinity of ornaments, the most distinguished of which was the statue of Jupiter with his golden thunderbolt, his sceptre, and crown. In the capitol also were a temple dedicated to Jupiter the Guardian, and another to Juno with the mint; and on the descent of the hill was the temple of Concord. This beautiful edifice contained the most sacred deposits of religion, such as the ancilia, the books of the Sibyls, &c. The capitol was burnt under Vitellius, and rebuilt under Vespasian. It was burnt a second time by lightning under Titus, and restored by Domitian. The modern name is Campidoglio.