Sweyn II Estridsson Ulfsson. (1018?-1076) was the King of Denmark from 1047 to 1076. He was the son of Ulf Thorgilsson and Estrid Margarete Svendsdatter , daughter of Sweyn I of Denmark. He fought Norway's King Harald Hardråde in a long war until 1064 when Harald relinquished his claims to Denmark. After that Sweyn begun to build a strong foundation for royal power through cooperation with the church. He completed the final partition of Denmark into dioceses, and corresponded with the Pope. Sweyn seems to have been able to read and write, and he is the source of much of our current knowledge about Denmark in the 9th and 10th centuries, having told the story of his ancestry to historian Adam of Bremen around 1070.

Sweyn Estridsson joined forces with Edgar Atheling when he attempted to regain the English throne from William the Conqueror. However after capturing York Sweyn accepted a payment from William to desert Edgar, who returned into exile in Scotland.

Sweyn's first marriage was to a girl to whom he was distantly related, and it was ordered by the Pope to dissolve the union, which he did, only to take one mistress after another during the rest of his life. Sweyn Estridsson fathered at least 19 children, probably more, and while none of them were born in wedlock, and none of their mothers are known, five of his numerous sons became kings after their father, beginning with Harald Hen in 1076 and ending with King Niels, who was murdered in 1134.

Sweyn is often considered to be Denmark's first medieval King. His line of male descendants effectively died out in 1387 when the young King Oluf III died, and the new King had to be found among the sons of his female descendants. His skeleton shows him to have been a tall, powerfully built man who walked with a limp.