This forum was built on the order of the emperor Trajan with the spoils of war from the conquest of Dacia, which ended in 106. The construction began between 105 and 107; according to the Fasti Ostienses the Forum was inaugurated in 112. Trajan's Column was erected and then inaugurated in 113.
To build this monumental complex, extensive excavations were required: workers eliminated a ridge connecting the Quirinal and Capitoline (Campidoglio) Hills. Over 300,000 cubic meters of soil and rock were excavated and dumped outside the Porta Collina.
It is possible that the excavations were initiated under Emperor Domitian, while the project of the Forum was completely attributed to the architect Apollodorus of Damascus, who also accompanied Emperor Trajan in the Dacian campaign.
During the time of the construction, several other projects took place: the construction of the Markets of Trajan, the renovation of Caesar's Forum (where the Basilica Argentaria was built) and the Temple of Venus Genetrix.