The forum was built on the order of Emperor Trajan with the spoils of war from the conquest of Dacia, which ended in 106. The Fasti Ostiensi (see Fasti) states that the Forum was inaugurated in 112, while Trajan's Column was inaugurated in 113.
To build this monumental complex, extensive excavations were required: workers eliminated the sides of the Quirinal and Capitoline (Campidoglio) Hills, which closed the valley occupied by the Imperial forums toward the Campus Martius.
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 Markets of Trajan were constructed, Caesar's Forum (where the Basilica Argentaria was built), and the Temple of Venus Genetrix were renovated.
The Forum was built from a vast stoa-lined piazza measuring 660 by 390 feet (200 x 120 m) with exedrae on two sides. The main entrance to the forum is on the southern side, a triumphal arch surmounted by a statue of Trajan in a six-horse chariot. The Basilica Ulpia lies at the north end of the piazza, which was cobbled with rectangular blocks of white marble and decorated by a large equestrian statue of Trajan. On either side of the piazza are markets, also housed by the exedrae.
North of the Basilica was a smaller piazza, with a temple dedicated to the deified Trajan on the far north side facing inwards. Directly north of the Basilica Ulpia on either side of the forum were two libraries, one housing Latin documents and the other Greek documents. Between the libraries was the 125-foot (38 m) Trajan's Column.
In the mid-4th century, Constantius II, while visiting Rome, was amazed by the huge equestrian statue of Trajan and by the surrounding buildings. The imperial visit and the landmarks of the forum were described by historian Ammianus Marcellinus.
In modern times only a section of the markets and the column of Trajan remain.
In the mid-9th century, the marble cobble blocks of the piazza were systematically taken for re-use, because of the good quality of the lime. At the same time, the pavement was restored in wrought as a sign that the piazza was still in use as a public space.