Sweyn II Estridsson (Old Norse: Sveinn Ástríðarson, Danish: Svend Estridsen) (c. 1019 – 28 April 1076) was King of Denmark from 1047 until his death in 1076. He was the son of Ulf Thorgilsson and Estrid Svendsdatter. He was married three times, and fathered 20 children or more out of wedlock, including the five future kings Harald III Hen, Canute IV the Saint, Oluf I Hunger, Eric I Evergood, and Niels.

He was courageous in battle, but did not have much success as a military commander. His skeleton reveals that he was a tall, powerfully built man who walked with a limp.

Sweyn was born in England, as the son of Ulf Thorgilsson and Estrid Svendsdatter, the daughter of king Sweyn I of Denmark and sister of king Canute the Great. Sweyn grew up a military leader, and served under king Anund Jacob of Sweden for a time. He pillaged the Elbe-Weser area in 1040, but was caught by the Archbishop of Hamburg-Bremen, who released him shortly thereafter.

Svend was made jarl under Danish king Harthacnut, and led a campaign for him against Norway, but was beaten by Magnus I of Norway. When Harthacnut died in 1042, Magnus claimed the Danish throne and made Svend his jarl of Jutland. In 1043, Sweyn fought for Magnus at the Battle of Lyrskov Heath at Hedeby, near the present-day border of Denmark and Germany. Sweyn won great reputation at Lyrskov Heath, and had the Danish nobles crown him king in Viborg in Jutland. He was defeated by Magnus on several occasions, and had to flee to Sweden. Eventually he managed to return and establish a foothold in Scania.

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