Decebalus (Ancient Greek: Δεκέβαλος, romanized: Dekebalos; r. 87 – 106 AD), sometimes referred to as Diurpaneus, was the last Dacian king. He is known for fighting three wars, with varying success, against the Roman Empire under two emperors. After raiding south across the Danube, he defeated a Roman invasion in the reign of Domitian, securing a period of independence during which Decebalus consolidated his rule.

When Trajan came to power, his armies invaded Dacia to weaken its threat to the Roman border territories of Moesia. Decebalus was defeated in 102 AD, and his own sister was abducted within this timeframe and forcibly wed into Roman nobility, causing some historians to infer that she was the ancestress of the usurper, Regalianus, who claimed to be a kinsman of Decebalus. He remained in power as a client king, but continued to assert his independence, leading to a final and overwhelming Roman invasion north of the Danube in 105 AD. Trajan reduced the Dacian capital Sarmizegetusa to ruins in 106 AD, absorbing some of Dacia into the Empire. Decebalus died by suicide via cutting his own throat to avoid capture.

After the death of Great King Burebista, Dacia split into four, then five smaller kingdoms. Nothing is known about Decebalus's youth or background. Decebalus appears to have risen to prominence in the court of the Dacian king Duras, who claimed authority over all Dacian territory. An ancient Dacian pot bearing the words “Decebalus per Scorilo” led to the suggestion that this might mean "Decebalus son of Scorilo".

According to Lucian Boia this suggestion was originally a "scholarly joke", but the theory has been considered plausible by several writers. It has been suggested that "Scorilo" may be identical to the "Coryllus" or "Scorillus" identified by Jordanes as a Dacian king prior to Duras. Duras may have been Decebalus's uncle, having taken over the throne by agnatic right on his brother's death.

In 85 AD the Dacian army began minor raids upon the heavily fortified Roman province of Moesia, located south of the Danube. In 86 King Duras ordered a more vigorous attack south into Moesia. Roman sources refer to the attack being led by "Diurpaneus" (or "Dorpaneus"). Many authors have taken this person to be Duras himself, and refer to him as "Duras-Diurpaneus". Other scholars argue that Duras and Diurpaneus are different individuals, or that Diurpaneus is identical to Decebalus. Recent sources take the view that "Diurpaneus" is most likely Decebalus.

The Dacians defeated and killed Oppius Sabinus, the governor of Moesia, forcing Domitian to deploy more troops to the area. Marcus Cornelius Nigrinus replaced Sabinus. Domitian took command to deal with the problem himself, arriving with his general, prefect of the Praetorian Guards, Cornelius Fuscus.

Domitian pushed back the Dacians from Moesia, then returned to Rome to celebrate a Triumph, leaving Fuscus in charge of the army. Fuscus advanced into Dacia, but his four or five legions suffered a major defeat when ambushed by the forces of Decebalus (the sources say "Diurpaneus" was in command, which might mean Decebalus or Duras). Two Roman legions (among which was the V Alaudae) were ambushed and defeated at a mountain pass the Romans called Tapae (widely known as the Iron Gates of Transylvania). Fuscus was killed, and Decebalus was crowned king after the ageing Duras abdicated.

Dio Cassius described Decebalus as follows: