Coenwulf (Old English: [ˈkøːnwuɫf]; also spelled Cenwulf, Kenulf, or Kenwulph; Latin: Coenulfus) was the king of Mercia from December 796 until his death in 821. He was a descendant of King Pybba, who ruled Mercia in the early 7th century. He succeeded Ecgfrith, the son of Offa; Ecgfrith only reigned for five months, and Coenwulf ascended the throne in the same year that Offa died. In the early years of Coenwulf's reign he had to deal with a revolt in Kent, which had been under Offa's control. Eadberht Præn returned from exile in Francia to claim the Kentish throne, and Coenwulf was forced to wait for papal support before he could intervene. When Pope Leo III agreed to anathematise Eadberht, Coenwulf invaded and retook the kingdom; Eadberht was taken prisoner, was blinded, and had his hands cut off. Coenwulf also appears to have lost control of the kingdom of East Anglia during the early part of his reign, as an independent coinage appears under King Eadwald. Coenwulf's coinage reappears in 805, indicating that the kingdom was again under Mercian control. Several campaigns of Coenwulf's against the Welsh are recorded, but only one conflict with Northumbria, in 801, though it is likely that Coenwulf continued to support the opponents of the Northumbrian king Eardwulf.

Coenwulf came into conflict with Archbishop Wulfred of Canterbury over the issue of whether laypeople could control religious houses such as monasteries. The breakdown in the relationship between the two eventually reached the point where the archbishop was unable to exercise his duties for at least four years. A partial resolution was reached in 822 with Coenwulf's successor, King Ceolwulf, but it was not until about 826 that a final settlement was reached between Wulfred and Coenwulf's daughter, Cwoenthryth, who had been the main beneficiary of Coenwulf's grants of religious property.

Coenwulf was succeeded by his brother, Ceolwulf; a post-Conquest legend claims that his son Cynehelm was murdered to gain the succession. Within two years Ceolwulf had been deposed, and the kingship passed permanently out of Coenwulf's family. Coenwulf was the last king of Mercia to exercise substantial dominance over other Anglo-Saxon kingdoms. Within a decade of his death, the rise of Wessex had begun under King Egbert, and Mercia never recovered its former position of power.

For most of the 8th century, Mercia was dominant among the Anglo-Saxon kingdoms south of the river Humber. Æthelbald, who came to the throne in 716, had established himself as the overlord of the southern Anglo-Saxons by 731. He was assassinated in 757, and was briefly succeeded by Beornred, but within a year Offa ousted Beornred and took the throne for himself. Offa's daughter Eadburh married Beorhtric of Wessex in 789, and Beorhtric became an ally thereafter. In Kent, Offa intervened decisively in the 780s, and at some point became the overlord of East Anglia, whose king, Æthelred, was beheaded on Offa's orders in 794.

Offa appears to have moved to eliminate dynastic rivals to the succession of his son, Ecgfrith. According to a contemporary letter from Alcuin of York, an English deacon and scholar who spent over a decade as a chief advisor at Charlemagne's court, "the vengeance of the blood shed by the father has reached the son"; Alcuin added, "This was not a strengthening of the kingdom, but its ruin." Offa died in July 796. Ecgfrith succeeded him but reigned for less than five months before Coenwulf came to the throne.

A significant corpus of letters dates from the period, especially from Alcuin, who corresponded with kings, nobles, and ecclesiastics throughout England. Letters between Coenwulf and the papacy also survive. Another key source for the period is the Anglo-Saxon Chronicle, a collection of annals in Old English narrating the history of the Anglo-Saxons. The Chronicle was a West Saxon production, however, and is sometimes thought to be biased in favour of Wessex. Charters dating from Coenwulf's reign have survived; these were documents granting land to followers or to churchmen and were witnessed by the kings who had the authority to grant the land. A charter might record the names of both a subject king and his overlord on the witness list appended to the grant. Such a witness list can be seen on the Ismere Diploma, for example, where Æthelric, son of king Oshere of the Hwicce, is described as a "subregulus", or subking, of Æthelbald.

According to the Anglo-Saxon Chronicle, Ecgfrith only reigned for 141 days. Offa is known to have died in 796, on either 26 July or 29 July, so Ecgfrith's date of death is either 14 December or 17 December of the same year. Coenwulf succeeded Ecgfrith as king. Coenwulf's father's name was Cuthberht, who may have been the same person as an ealdorman of that name who witnessed charters during the reign of Offa. Coenwulf is also recorded as witnessing charters during Offa's reign. According to the genealogy of Mercian kings preserved in the Anglian collection Coenwulf was descended from a brother of Penda named Cenwealh, of whom there is no other record. It is possible that this refers to Cenwealh of Wessex, who was married to (and later repudiated) a sister of Penda.

Coenwulf's kin may have been connected to the royal family of the Hwicce, a subkingdom of Mercia around the lower river Severn. It appears that Coenwulf's family were powerful, but they were not of recent Mercian royal lineage. A letter written by Alcuin to the people of Kent in 797 laments that "scarcely anyone is found now of the old stock of kings". Eardwulf of Northumbria had, like Coenwulf, gained his throne in 796, so Alcuin's meaning is not clear, but it may be that he intended it as a slur on Eardwulf or Coenwulf or on both. Alcuin certainly held negative views of Coenwulf, regarding him as a tyrant and criticising him for putting aside one wife and taking another. Alcuin wrote to a Mercian nobleman to ask him to greet Coenwulf peaceably "if it is possible to do so", implying uncertainty about Coenwulf's policy towards the Carolingians.