Faustina (fl. 361 – after 366) was an Empress of the Roman Empire and third wife of Emperor Constantius II. The main source for her biography is the account of historian Ammianus Marcellinus. Her origins and other names are unknown.
Constantius married her in Antioch in 361, after the death of his second wife, Eusebia in 360. Ammianus simply reports that the marriage took place while Constantius was wintering in Antioch, taking a break from the ongoing Roman–Persian Wars. "At that same time Constantius took to wife Faustina, having long since lost Eusebia".
She was pregnant when Constantius died on 3 November 361 and later gave birth to their posthumous daughter, Flavia Maxima Constantia, the only child of the emperor. Constantia later married Emperor Gratian.
On 28 September 365 Faustina was present when Procopius received the insignia of the imperial rites in Constantinople. Faustina and her little daughter's presence suggested that Procopius was the rightful heir of the Constantinian dynasty which was still held in reverence.
Ammianus considers that Procopius having Faustina and Constantia by his side increased the loyalty of the people to his cause:
"Valens called forth his troops and joining with him Lupicinus and a strong force of auxiliaries, he hastened to Pessinus, formerly a town of Phrygia, now of Galatia. Having safely garrisoned this place in order to suffer no surprise in those parts, he marched along the foot of the lofty mountain called Olympus, and over rocky paths, towards Lycia, planning to attack Gomoarius, while he loitered there half asleep. But he [Valens] was met with general and obstinate resistance, for this reason in particular — that his enemy (as has been mentioned) both on the march and when they were almost in battle array, carried about with him in a litter the little daughter of Constantius, and her mother Faustina; and thereby had inflamed the passions of the soldiers to fight more bravely in defence of the imperial stock, with which he claimed that he himself was connected."
After the Battle of Thyatira and the fall of Procopius in 366, Faustina passes out of sight.