Marcus Claudius Tacitus (died June 276) was Roman emperor from 275 to 276. During his short reign he campaigned against the Goths and the Heruli, for which he received the title Gothicus Maximus. 

After the assassination of Aurelian, the army, apparently showing remorse towards its role in the death of the beloved emperor, relinquished the right of choosing his successor to the Senate.  After a few weeks, the throne was offered to the aged Princeps Senatus, Tacitus.

According to the Historia Augusta, Tacitus, after ascertaining the sincerity of the Senate's regard for him, accepted their nomination on 25 September 275,  and the choice was cordially ratified by the army.  If true, Tacitus would have been the last emperor elected by the Senate.  However, it's possible that much of this narrative is fictitious, as Zosimus and Zonaras report that Tacitus was actually proclaimed by the army without any intervention of the Senate.

Amongst the highest concerns of the new reign was the restoration of the ancient Senatorial powers. He granted substantial prerogatives to the Senate, securing to them by law the appointment of the emperor, of the consuls, and the provincial governors, as well as supreme right of appeal from every court in the empire in its judicial function, and the direction of certain branches of the revenue in its long-abeyant administrative capacity.  Probus respected these changes, but after the reforms of Diocletian in the succeeding decades not a vestige would be left of them.

Next he moved against the barbarian mercenaries that had been gathered by Aurelian to supplement Roman forces for his Eastern campaign.  These mercenaries had plundered several towns in the Eastern Roman provinces after Aurelian had been murdered and the campaign cancelled.  His half-brother, the Praetorian Prefect Florian, and Tacitus himself won a victory against these tribes, among which were the Heruli, gaining the emperor the title Gothicus Maximus.

On his way back to the west to deal with a Frankish and Alamannic invasion of Gaul, according to Aurelius Victor, Eutropius and the Historia Augusta, Tacitus died of fever at Tyana in Cappadocia around June 276, after a rule of just over 6 months. In a contrary account, Zosimus claims he was assassinated, after appointing one of his relatives to an important command in Syria.