Marcus Aurelius Claudius "Gothicus" (10 May 214 – August/September 270), also known as Claudius II, was Roman emperor from 268 to 270. During his reign he fought successfully against the Alemanni and decisively defeated the Goths at the Battle of Naissus. He died after succumbing to a "pestilence", possibly the Plague of Cyprian that had ravaged the provinces of the Empire.   Claudius was an Illyrian and came from Dalmatia or Illyricum, although it is possible that his place of birth was the region of Dardania in Moesia Superior. According to the fourth-century Epitome de Caesaribus, he was thought to be a bastard son of Gordian II,  although this is doubted by some historians.

The Historia Augusta refers to him as a member of the gens Flavia, likely an attempt to further connect him with the future emperor Flavius Valerius Constantius.

Before coming to power, Claudius served with the Roman army, where he had a successful career and secured appointments to the highest military posts. During the reign of Decius (249–251) he served as a military tribune. In this post, Claudius was sent to defend Thermopylae, in connection with which the governor of Achaea was ordered to send him 200 Dardanian soldiers, 60 horsemen, 60 Cretan archers, and a thousand well-armed recruits. However, there is no evidence that the Goths who invaded at that time threatened the region, since their invasion did not extend beyond the middle Balkans. Most likely the message of the "History of the Augustus" is an anachronism, since it is known that the garrison at Thermopylae appeared in 254. Historian François Pashau suggests that this passage was invented in order to contrast the successful pagan commander Claudius with the unlucky Christian generals who allowed the ruin of Greece by the Gothic leader Alaric I in 396. In addition, Trebellius Pollio reveals that Decius rewarded Claudius after he demonstrated his strength while fighting another soldier at the Games of Mars.

His troops then proclaimed him emperor amid charges, never proven, that he murdered his predecessor Gallienus. However, he soon showed himself to be less than bloodthirsty, as he asked the Roman Senate to spare the lives of Gallienus's family and supporters. He was less magnanimous toward Rome's enemies and it was to this that he owed his popularity.

It is possible Claudius gained his position and the respect of the soldiers by being physically strong and especially cruel. A legend tells of Claudius knocking out a horse's teeth with one punch. When Claudius performed as a wrestler in the 250s, he supposedly knocked out the teeth of his opponent when his genitalia had been grabbed in the match.

Claudius, like Maximinus Thrax before him, was of barbarian birth. After a period of failed aristocratic Roman emperors following Maximinus's death, Claudius was the first in a series of tough "soldier emperors" who would eventually restore the Empire after the Crisis of the Third Century.

The death of Gallienus was surrounded by conspiracy and betrayal, as were many emperors' deaths. Different accounts of the incident have been recorded, but they agree that senior officials wanted Gallienus dead. According to two accounts, the prime conspirator was Aurelius Heraclianus, the Praetorian Prefect. One version of the story tells of Heraclianus bringing Claudius into the plot while the account given by the Historia Augusta exculpates the soon-to-be emperor and adds the prominent general Lucius Aurelius Marcianus into the plot. The removal of Claudius from the conspiracy may be due to his later role as the progenitor of the house of Constantine, a fiction of Constantine's time, and suggests that the original version from which these two accounts derive was current prior to the reign of Constantine. It was written that while sitting down at dinner, Gallienus was told that Aureolus and his men were approaching the camp. Gallienus rushed to the front lines, ready to give orders, when he was struck down by a commander of his cavalry. In a different and more controversial account, Aureolus forges a document in which Gallienus appears to be plotting against his generals and makes sure it falls into the hands of the emperor's senior staff. In this plot, Aurelian is added as a possible conspirator. The tale of his involvement in the conspiracy might be seen as at least partial justification for the murder of Aurelian himself under circumstances that seem remarkably similar to those in this story.

Whichever story is true, Gallienus was killed in the summer of 268, probably between July and October, and Claudius was chosen by the army outside of Milan to succeed him. Accounts tell of people hearing the news of the new emperor, and reacting by murdering Gallienus's family members until Claudius declared he would respect the memory of his predecessor. Claudius had the deceased emperor deified and buried in a family tomb on the Appian Way. The traitor Aureolus was not treated with the same reverence, as he was killed by his besiegers after a failed attempt to surrender.

At the time of Claudius's accession, the Roman Empire was in serious danger from several incursions, both inside and outside its borders. The most pressing of these was an invasion of Illyricum and Pannonia by the Goths. Although Gallienus had already inflicted some damage on them at the Battle of Nestus, Claudius, not long after being named emperor, followed this up by winning his greatest victory, and one of the greatest in the history of Roman arms.

At the Battle of Naissus, Claudius and his legions routed a huge Gothic army. Together with his cavalry commander, the future Emperor Aurelian, the Romans took thousands of prisoners and destroyed the Gothic cavalry as a force. The victory earned Claudius his surname of "Gothicus" (conqueror of the Goths). The Goths were soon driven back across the Danube River by Aurelian, and nearly a century passed before they again posed a serious threat to the empire.

Around the same time, the Alamanni had crossed the Alps and attacked the empire. Claudius responded quickly, routing the Alamanni at the Battle of Lake Benacus in the late fall of 268, a few months after the Battle of Naissus.  For this he was awarded the title of "Germanicus Maximus."  He then turned on the Gallic Empire, ruled by a pretender for the past eight years and encompassing Britain, Gaul, and the Iberian Peninsula. He won several victories and soon regained control of Hispania and the Rhone river valley of Gaul.  This set the stage for the later destruction of the Gallic Empire under Aurelian.

The victories of Claudius over the Goths would not only make him a hero in Latin tradition, but an admirable choice as an ancestor for Constantine I, who was born at Naissus, the site of Claudius's victory in 269. Claudius is also held in high esteem by Zonaras, whose Greek tradition seems to have been influenced by Latin. For Zosimus, a more reasoned contemporary view shows Claudius as less grand. Claudius's successes in the year 269 were not continued in his next year as emperor. As the Scythians starved in the mountains or surrendered, the legions pursuing them began to see an epidemic spreading throughout the men. Also, Claudius's unwillingness to do anything at the siege of Autun likely provoked a quarrel with Zenobia.

Claudius did not live long enough to fulfill his goal of reuniting all the lost territories of the empire. Late in 269 he had travelled to Sirmium and was preparing to go to war against the Vandals, who were raiding in Pannonia. However, he fell victim to the Plague of Cyprian (possibly smallpox), and died early in 270.  Before his death, he is thought to have named Aurelian as his successor, though Claudius's brother Quintillus briefly seized power. The Senate immediately deified Claudius as "Divus Claudius Gothicus."