Æthelberht (/ˈæθəlbərt/; also Æthelbert, Aethelberht, Aethelbert or Ethelbert; Old English: Æðelberht [ˈæðelberˠxt]; c. 550 – 24 February 616) was King of Kent from about 589 until his death. The eighth-century monk Bede, in his Ecclesiastical History of the English People, lists him as the third king to hold imperium over other Anglo-Saxon kingdoms. In the late ninth century Anglo-Saxon Chronicle, he is referred to as a bretwalda, or "Britain-ruler". He was the first Anglo-Saxon king to convert to Christianity.

Æthelberht was the son of Eormenric, succeeding him as king, according to the Chronicle. He married Bertha, the Christian daughter of Charibert I, king of the Franks, thus building an alliance with the most powerful state in contemporary Western Europe; the marriage probably took place before he came to the throne. Bertha's influence may have led to Pope Gregory I's decision to send Augustine as a missionary from Rome. Augustine landed on the Isle of Thanet in east Kent in 597. Shortly thereafter, Æthelberht converted to Christianity, churches were established, and wider-scale conversion to Christianity began in the kingdom. He provided the new church with land in Canterbury, thus helping to establish one of the foundation stones of English Christianity.

Æthelberht's law for Kent, the earliest written code in any Germanic language, instituted a complex system of fines; the law code is preserved in the Textus Roffensis. Kent was rich, with strong trade ties to the Continent, and Æthelberht may have instituted royal control over trade. Coinage probably began circulating in Kent during his reign for the first time since the Anglo-Saxon settlement. He later came to be regarded as a saint for his role in establishing Christianity among the Anglo-Saxons. His feast day was originally 24 February but was changed to 25 February.

In the fifth century, raids on Britain by continental peoples had developed into full-scale migrations. The newcomers are known to have included Angles, Saxons, Jutes and Frisians, and there is evidence of other groups as well. These groups captured territory in the east and south of England, but at about the end of the fifth century, a British victory at the battle of Mount Badon (Mons Badonicus) halted the Anglo-Saxon advance for fifty years. From about 550, however, the British began to lose ground once more, and within twenty-five years it appears that control of almost all of southern England was in the hands of the invaders.

Anglo-Saxons probably conquered Kent before Mons Badonicus. There is both documentary and archaeological evidence that Kent was primarily colonised by Jutes, from the southern part of the Jutland peninsula. According to legend, the brothers Hengist and Horsa landed in 449 as mercenaries for a British king, Vortigern. After a rebellion over pay and Horsa's death in battle, Hengist established the Kingdom of Kent. Some historians now think the underlying story of a rebelling mercenary force may be accurate; most now date the founding of the kingdom of Kent to the middle of the fifth-century, which is consistent with the legend. This early date, only a few decades after the departure of the Romans, also suggests that more of Roman civilization may have survived into Anglo-Saxon rule in Kent than in other areas.

Overlordship was a central feature of Anglo-Saxon politics which began before Æthelberht's time; kings were described as overlords as late as the ninth century. The Anglo-Saxon invasion may have involved military coordination of different groups within the invaders, with a leader who had authority over many different groups; Ælle of Sussex may have been such a leader. Once the new states began to form, conflicts among them began. Tribute from dependents could lead to wealth. A weaker state also might ask or pay for the protection of a stronger neighbour against a warlike third state.

Sources for this period in Kentish history include the Ecclesiastical History of the English People, written in 731 by Bede, a Northumbrian monk. Bede was interested primarily in England's Christianization. Since Æthelberht was the first Anglo-Saxon king to convert to Christianity, Bede provides more substantial information about him than about any earlier king. One of Bede's correspondents was Albinus, abbot of the monastery of St. Peter and St. Paul (subsequently renamed St. Augustine's) in Canterbury. The Anglo-Saxon Chronicle, a collection of annals assembled c. 890 in the kingdom of Wessex, mentions several events in Kent during Æthelberht's reign. Further mention of events in Kent occurs in the late sixth century history of the Franks by Gregory of Tours. This is the earliest surviving source to mention any Anglo-Saxon kingdom. Some of Pope Gregory the Great's letters concern the mission of St. Augustine to Kent in 597; these letters also mention the state of Kent and its relationships with neighbours. Other sources include regnal lists of the kings of Kent and early charters (land grants by kings to their followers or to the church). Although no originals survive from Æthelberht's reign, later copies exist. A law code from Æthelberht's reign also survives.

According to Bede, Æthelberht was descended directly from Hengist. Bede gives the line of descent as follows: "Ethelbert was son of Irminric, son of Octa, and after his grandfather Oeric, surnamed Oisc, the kings of the Kentish folk are commonly known as Oiscings. The father of Oeric was Hengist." An alternative form of this genealogy, found in the Historia Brittonum among other places, reverses the position of Octa and Oisc in the lineage. The first of these names that can be placed historically with reasonable confidence is Æthelberht's father, whose name now usually is spelled Eormenric. The only direct written reference to Eormenric is in Kentish genealogies, but Gregory of Tours does mention that Æthelberht's father was the king of Kent, though Gregory gives no date. Eormenric's name provides a hint of connections to the kingdom of the Franks, across the English channel; the element "Eormen" was rare in names of the Anglo-Saxon aristocracy, but much more common among Frankish nobles. One other member of Æthelberht's family is known: his sister, Ricole, who is recorded by both Bede and the Anglo-Saxon Chronicle as the mother of Sæberht, king of the East Saxons (i.e., Essex).