Legio I Adiutrix ("assistant"), was a Roman legion formed in 68 AD, possibly by Galba under orders of Nero. The last record mentioning the Adiutrix is in 444 AD, when it was stationed at Szöny (Brigetio), in the Roman province of Pannonia. The emblem of the legion was a Capricorn, used along with the winged horse Pegasus.

In the confusing Year of the four emperors, the legion presumably fought first for Galba, then in Otho's army in the Battle of Bedriacum, where this emperor was defeated by Vitellius. By the year 70 AD, the legion was fighting in the Batavian rebellion.

The city of Mainz (Moguntiacum) is the legion's first known base camp, shared with Legio XIV Gemina, where they attended mainly building activities. In 83 AD they fought the Germanic wars against the Chatti, a German tribe living across the Rhine, under the command of Emperor Domitian. After that they were transferred to the Danubian army stationed in the Roman province of Pannonia, to fight the Dacians.

Following the murder of Domitian in 96 AD, the Adiutrix, along with the Danubian army, played an important role in Roman politics, forcing Nerva to adopt Trajan as his successor. When Trajan became emperor, he gave the legion the cognomen pia fidelis ("loyal and faithful") to acknowledge their support. Between 101 and 106 AD, under the new emperor's command, the Adiutrix, along with Legio IV Flavia felix and Legio XIII Gemina, conquered Dacia and occupied the newly formed province. Trajan also used his pia fidelis in the campaign against Parthia (115–117 AD), but they were sent back to Pannonia by his successor emperor Hadrian.

During the next decades, the Adiutrix remained in the Danube frontier. Between 171 and 175, the commander was Pertinax, emperor for a brief period in 193. When Septimius Severus became emperor, the Adiutrix was among his supporters, following him in the march for Rome.

In the next decades, the main base was again Pannonia, but they played a part in several Parthian wars, namely the campaigns of 195 and 197–198 of Septimius Severus, 215–217 led by Caracalla and 244 by Gordian III.

The legion received the cognomen pia fidelis bis ("twice loyal and faithful") and constans ("reliable"), sometime in the 3rd century.

See also: List of Roman legions