Marcus Ulpius Traianus was born on September 18, 53, in the Roman province of Hispania Baetica (in what is now Andalusia in modern Spain), a province that was thoroughly Romanized and called southern Hispania, in the city of Italica (now in the outskirts of Seville), where the Italian families were paramount.

He is the son of Marcia and Marcus Ulpius Traianus, a prominent senator and general from the gens Ulpia.

Trajan himself is just one of many well-known Ulpii in a line that will continue long after his own death.

His elder sister is Ulpia Marciana and his niece was Salonina Matidia.

The patria of the Ulpii is Italica, in Spanish Baetica, where their ancestors late in the third century BCE had settled.

He had risen as a young man through the ranks of the Roman army, serving in some of the most contentious parts of the Empire's frontier.

Trajan's father was in 76–77 Governor of Syria (Legatus pro praetore Syriae), where Trajan himself had remained as Tribunus legionis.

The governor of Germania Superior, Lucius Antonius Saturninus, and his two legions at Mainz, Legio XIV Gemina and Legio XXI Rapax, on January 1, 89,  revolt against the Empire with the aid of the Chatti.

The precise cause for the rebellion is uncertain, although it appears to have been planned well in advance.

The Senatorial officers may have disapproved of Domitian's military strategies, such as his decision to fortify the German frontier rather than attack, as well as his recent retreat from Britain, and finally the disgraceful policy of appeasement towards Decebalus.

The uprising is in any case strictly confined to Saturninus' province, and quickly detected once the rumor spreads across the neighboring provinces.

The governor of Germania Inferior, Lappius Maximus, moves to the region at once, assisted by the procurator of Rhaetia, Titus Flavius Norbanus.

Trajan is summoned from Spain, while Domitian himself comes from Rome with the Praetorian Guard.

By a stroke of luck, a thaw prevents the Chatti from crossing the Rhine and coming to Saturninus' aid.

The rebellion is crushed within twenty-four days and its leaders at Mainz savagely punished.

Trajan afterwards burns Saturninus' letters in an attempt to avoid implicating others.

Domitian has numerous others executed with Saturninus, however, displaying their heads on the rostra at Rome.

The Legio XXI is sent to the front in Pannonia, and Domitian passes a law prohibiting two legions from sharing the same camp.