Aerarium (from Latin "aes", in its derived sense of "money") was the name (in full, "aerarium stabulum" - treasure-house) given in Ancient Rome to the public treasury , and in a secondary sense to the public finances.

The treasury contained the monies and accounts of the state finances, and also the standards of the legions; the public laws engraved on brass, the decrees of the Senate and other papers and registers of importance.

These public treasures were deposited in the temple of Saturn , on the eastern slope of the Capitoline Hill, and, during the republic, were in charge of the urban quaestors, under the superintendence and control of the senate.

This arrangement continued (except for the year 43 BC, when no quaestors were chosen) until 28 BC, when Augustus transferred the aerarium to two praefecti aerarii, chosen annually by the senate from ex-praetors. In 23 these were replaced by two praetors (praetores aerarii or ad aerarium), selected by lot during their term of office; Claudius in 44 restored the quaestors, but nominated by the emperor for three years, for whom Nero in 56 substituted two ex-praetors, under the same conditions.

In addition to the common treasury, supported by the general taxes and charged with the ordinary expenditure, there was a special reserve fund, also in the temple of Saturn, the aerarium's sanctum (or sanctius), probably originally consisting of the spoils of war , afterwards maintained chiefly by a 5% tax on the value of all manumitted slaves, this source of revenue being established by a lex Manila in 357. This fund was not to be touched except in cases of extreme necessity.

Under the emperors, the senate continued to have at least the nominal management of the aerarium, while the emperor had a separate exchequer, called fiscus. However, after a time, as the power of the emperors increased and their jurisdiction extended until the senate existed only in form and name, this distinction virtually ceased.

Besides creating the fiscus, Augustus also established in AD 6 a military treasury (aerarium militare), containing all monies raised for and appropriated to the maintenance of the army, including a pension fund for disabled soldiers. It was largely endowed by the emperor himself and supported by the proceeds of the tax on public sales and the succession duty. Its administration was in the hands of three praefecti aerarii militaris, at first appointed by lot, but afterwards by the emperor, from senators of praetorian rank, for three years. The later emperors had a separate aerarium privatum, containing the monies allotted for their own use, distinct from the fiscus, which they administered in the interests of the empire.

The tribuni aerarii have been the subject of much discussion. They are supposed by some to be identical with the curatores tribuum, and to have been the officials who, under the Servian organization, levied the war-tax (tributum) in the tribes and the poll-tax on the aerarii. They also acted as paymasters of the equites and of the soldiers on service in each tribe. By the lex Amelia (70 BC) the list of judices was composed, in addition to senators and equites, of tribuni aerarii. Whether these were the successors of the above, or a new order closely connected with the equites, or even the same as the latter, is uncertain.

According to Mommsen, they were persons who possessed the equestrian census, but no public horse. They were removed from the list of judices by Caesar, but replaced by Augustus. According to Madvig , the original tribuni aerarii were not officials at all, but private individuals of considerable means, quite distinct from the curatores tribuum, who undertook certain financial work connected with their own tribes. Then, as in the case of the equites, the term was subsequently extended to include all those who possessed the property qualification that would have entitled them to serve as tribuni aerarii.