Commodus (/ˈkɒmədəs/; Latin: [ˈkɔmmɔdʊs]; 31 August 161 – 31 December 192) was Roman emperor from 177 to 192, first serving as nominal co-emperor under his father Marcus Aurelius and then ruling alone from 180. Commodus's sole reign is commonly thought to mark the end of the Pax Romana, a golden age of peace and prosperity in the history of the Roman Empire.

Commodus accompanied his father during the Marcomannic Wars in 172 and on a tour of the Eastern provinces in 176. The following year, he became the youngest emperor and consul up to that point, at the age of 16. His solo reign saw less military conflict than that of Marcus Aurelius, but internal intrigues and conspiracies abounded, goading Commodus to an increasingly dictatorial style of leadership. This culminated in his creating a deific personality cult, including his performances as a gladiator in the Colosseum. Throughout his reign, Commodus entrusted the management of affairs to his palace chamberlain and praetorian prefects, namely Saoterus, Perennis, and Cleander.

Commodus was assassinated by the wrestler Narcissus in 192, ending the Nerva–Antonine dynasty. He was succeeded by Pertinax, the first claimant in the tumultuous Year of the Five Emperors.

Lucius Aelius Aurelius Commodus was born on 31 August AD 161 in Lanuvium, near Rome. He was the son of the reigning emperor, Marcus Aurelius, and Aurelius' first cousin, Faustina the Younger, the youngest daughter of Emperor Antoninus Pius, who had died only a few months before. Commodus had a twin brother, Titus Aurelius Fulvus Antoninus, who died in 165. On 12 October 166, Commodus was made caesar together with his younger brother, Marcus Annius Verus. The latter died in 169 having failed to recover from an operation, which left Commodus as Marcus Aurelius's sole surviving son.

He was looked after by his father's physician, Galen, who treated many of Commodus's common illnesses. Commodus received extensive tutoring from a multitude of teachers with a focus on intellectual education. Among his teachers, Onesicrates, Antistius Capella, Titus Aius Sanctus, and Pitholaus are mentioned.

Commodus is known to have been at Carnuntum, the headquarters of Marcus Aurelius during the Marcomannic Wars, in 172. It is presumed that there, on 15 October 172, he was given the victory title Germanicus, in the presence of the army. The title suggests Commodus was present at his father's victory over the Marcomanni. On 20 January 175, Commodus entered the College of Pontiffs, the starting point of a career in public life.

In 175, Avidius Cassius, Governor of Syria, declared himself emperor following rumours that Marcus Aurelius had died. Having been accepted as emperor by Syria, Palestina and Egypt, Cassius carried on his rebellion even after it had become obvious Marcus was still alive. During the preparations for the campaign against Cassius, Commodus assumed his toga virilis on the Danubian front on 7 July 175, thus formally entering adulthood. Cassius, however, was killed by one of his centurions before the campaign against him could begin. Commodus subsequently accompanied his father on a lengthy trip to the Eastern provinces, during which he visited Antioch. The Emperor and his son then travelled to Athens, where they were initiated into the Eleusinian Mysteries. They then returned to Rome in the autumn of 176.

Marcus Aurelius was the first emperor since Vespasian to have a legitimate biological son, though he himself was the fifth in the line of the so-called Five Good Emperors, also known as the Adoptive Emperors, each of whom had adopted his successor. Commodus was the first (and until 337, the only) emperor "born in the purple," meaning during his father's reign.