Silbannacus was an obscure Roman emperor or usurper during the Crisis of the Third Century. Silbannacus is not mentioned in any contemporary documents and his existence was forgotten until the 20th century, when two coins bearing his name were discovered, the first in the 1930s and the second in the 1980s. His unusual name suggests that he might have been of Gallic descent.

As the only known evidence for his existence is the two coins, the exact time and extent of Silbannacus's rule is not known. Based on the design of the coin and its silver content, Silbannacus was most likely concurrent with the reigns of Philip the Arab (r. 244–249), Decius (r. 249–251), Trebonianus Gallus (r. 251–253), Aemilian (r. 253), or Valerian (r. 253–260). The two most prevalent ideas are the older hypothesis, that Silbannacus was a usurper in Gaul during the reign of Philip the Arab, at some point between 248 and 250, and the newer hypothesis, based on the design of the second coin, that Silbannacus was a briefly reigning legitimate emperor, holding Rome between the death of Aemilian and the arrival of Valerian.

The style of the second coin of Silbannacus appears to copy the design used on the coins of the emperor Aemilian (r. 253), which suggests that Silbannacus ruled later than the time of Philip, possibly around the time of Aemilian's short reign

253 was a turbulent year and many of the events that took place are obscure due to a lack of surviving sources. Aemilian's predecessor was Trebonianus Gallus (r. 251–253) and Aemilian had been proclaimed emperor by his troops after winning a victory against the Goths by the Danube. Gallus ordered the general Valerian to defeat the usurper, but Aemilian quickly reached Italy and overthrew Gallus. Aemilian's reign would be cut short when Valerian rebelled against him within weeks. Aemilian departed Rome to battle Valerian but was assassinated by his soldiers before a battle could take place. As features of the second coin are similar to features of coins minted at Rome, it is possible that Silbannacus was not an usurper in Gaul, but a briefly reigning ruler of the Roman capital.

Per the British historian Kevin Butcher, one possibility is that Silbannacus was an officer of Aemilian, who in the aftermath of Aemilian's death secured Rome and tried to rally against Valerian. If this is true, Silbannacus would have been unsuccessful, as Valerian took control of Rome shortly after Aemilian's death. That the coins of Silbannacus have both been found in Gaul does not discredit the idea that he ruled in Rome: currency moved around in the empire and there exists a traceable line of movement of coins from the capital to the Rhine frontier. Before the suggestion that the first coin was minted in Gaul was made, Mattingly had initially written that it was similar to the coins produced for Philip the Arab at Rome. If he ruled the capital, which would require support from the Roman Senate, Silbannacus may be counted as a legitimate, albeit ephemeral, emperor, rather than a failed usurper. Silbannacus as a ruler in Rome is supported as the most likely option by some historians,