Svend I "Forkbeard" (Svend Otto Haraldsen; Danish: Svend Tveskæg) (c. 960 – February 3, 1014). Sweyn succeeded his father Harold I "Blåtand" (Bluetooth) as king of Denmark, probably in late 986 or early 987. Sweyn had coins made with his likeness, being the first Danish king to do so. The inscription read "Zven, Rex ad Dener" which translates as "Sweyn, king of Danes". The year of his birth is unknown, but he is believed to have been born before his father accepted Christianity in the early or mid-960s. When the royal family converted Sven is said to have been given the Christian name Otto in honour of the German emperor. Sweyn is rarely recorded as having used this name though, and the inscriptions on his coinage and fact that he was accepted by the English Witan as king Sweyn would seem to corroborate this.

11th century historian Adam of Bremen, whose Gesta Hammaburgensis Ecclesiae Pontificum is strongly anti-Sweyn, claims that Sweyn Forkbeard was deposed by king Eric the Victorious of Sweden, who ruled Denmark until his death in 994 or 995. This has never been proven, but Adam can be cited as a secondary source. Adam's writings regarding Sweyn and his father may be compromised by Adam's desire to emphasize Sweyn's father, Harold, as a candidate for sainthood, and he claims that Sven, who was baptized along with his father, was a heathen. This may have been true, much of Scandinavia was pagan at the time, though there is no data, the German and French records support that Harold Bluetooth was baptized.

According to Adam, Sweyn was punished by God for supposedly leading the uprising which led to king Harold's death, and had to spend "fourteen years" abroad, perhaps a biblical reference from an ecclesiastical writer. Adam puports that Sweyn was shunned by all those with whom he sought refuge, but was finally allowed to live for a while in Scotland. The Scottish king at the time was apparently known in Europe as a heathen and a murderer, and Adam's intention is obviously to show that Sweyn belonged with heathens and murderers and couldn't rule a Christian country. He only achieves success as a ruler once he accepts Christ as his saviour. There are several major problems with a fourteen year "exile" in Scotland. Primarily, if Sweyn was exiled for fourteen years from his father's death in 985, the would be about c.1000. By 1000 it is stated that Sweyn has been ruling Denmark for about Fourteen years and Norway had been subjugated into a vassal state. Secondly, Adam of Bremen claims "the Scottish king at this time was a heathen"" however, in fourteen years from 995 there were five kings, Kenneth II, Olaf, Constantine III,the bald, Kenneth III and Giric II. It is unclear which king Adam meant, but the Scottish throne was constantly disputed, and the leading rival was Kenneth II, a man who is recognized as a Catholic and leading bureaucrat of his age. Certainly not a heathen murderer.

No other western European source maintains Adam's suggestions, however, and while some sort of conflict between Danes and Swedes almost certainly took place during Sweyn's reign, the idea of him being deposed has little foundation. And most historical sources agree that Sven subjugated Norway and created a vassal state, using both Danish and Norwegian forces in a combined assault on England. Many sources consider Olaf Skötkuning as the first ruler of a modern state of sweden, in which case Erik the victorious would not have ruled a nation state.

Whether King Sweyn was a heathen or not, he did enlist priests and bishops from England rather than from Hamburg, and this must have given Adam of Bremen further cause to dislike him. It also may have been because there were ample converted priests of a danish origin from the Danelaw in England, while Sweyn really had few connections to Germany or its priests. Sweyn must have known that once the archbishop of Hamburg-Bremen gained influence in Denmark, the German emperor Otto II would not be far behind; his Slavic neighbours to the south-east (balkans) had all but been annexed by Germany once Otto's father Otto I had divided their lands into bishoprics and put them under the "care" of the Holy Roman emperor. Sweyn may have envisaged the same happening to his own territory.

Following the death of Norway's king Olaf I Tryggvason in the Battle of Svolder, Sven established Danish control over most of Norway, with Erik Håkonsson, Earl as his vassal. Sweyn was almost certainly involved in the raids against England in 1003-1005, 1006-1007, and 1009-1012, following the St. Brice's Day massacre of England's Danish inhabitants in November 1002, recorded in the chronicles of John of Wallingford. Sven is thought to have had a personal interest in these due to his sister, Gunhilde, being amongst the victims. The massacre was large-scale ethnic cleansing of the Danish in England by Ethelred II the Unready. Sven acquired massive sums of Danegeld, and in 1013 personally led the Danish fleet in a full-scale invasion.

The contemporary Laud Chronicle states that "before the month of August came king Sweyn with his fleet to Sandwich. He went very quickly about East Anglia into the Humber's mouth, and so upward along the Trent till he came to Gainsborough. Eorl Uhtred and all Northumbria quickly bowed to him, as did all the folk of Lindsey, then the folk of the Five Boroughs. (...) He was given hostages from each shire. When he understood that all the people had submitted to him, he bade that his force should be provisioned and horsed; he went south in full force, and entrusted his ships and the hostages to his son Cnut. After he came over Watling Street, they worked the most evil that a force might do. They went to Oxford, and the town-dwellers soon bowed to him, and gave hostages. From there they went to Winchester, and did the same, then eastward to London."

But the Londoners are said to have destroyed the bridges that spanned the river Thames ("London Bridge is falling down"), and Sweyn suffered heavy losses and had to withdraw. The chronicles tell that "king Sven went from there to Wallingford, over the Thames to Bath, and stayed there with his troops; ealdorman Aethelmaer came, and the western thegns with him. They all bowed to Sweyn and gave hostages."

London had withstood the assault of the Danish army, but the city was now alone, isolated within a country which had completely surrendered. Sven Forkbeard was accepted as King of England following the flight to Normandy of king Ethelred the Unready in late 1013. With the acceptance of the Witan, London had finally surrendered to him, and he was declared "king" on Christmas day.
Sweyn based himself in Gainsborough, Lincolnshire, and began to organize his vast new kingdom, but he died there on February 3rd 1014, having ruled England unopposed for only five weeks. His body was subsequently returned to Denmark. He was succeeded as King of Denmark by his elder son, Harold II; the Danish fleet proclaimed his younger son Canute as King of England, but they and he returned to Denmark, with Æthelred being restored. Later, Canute ruled in Denmark, England, Norway and some parts of northern Germany.

Sweyn Forkbeard's nickname, which was probably used during his lifetime, refers to a long, pitchfork-like moustache, a "tjuge" in Old Norse, not to a full beard. Such a moustache was fashionable at the time, particularly in England.