Ulf Jarl in the context of Jarl (title)

Estrid Svendsdatter of Denmark (also known as Estrith or Astrith; sometimes called Margaret; fl. 1017–1032; c. 990s – after 1057 and before 1073) was a prominent Danish princess and titular queen of the Jelling dynasty, half-sister of Cnut the Great and wife of the magnate Ulf Jarl. She was the mother of Sweyn II Estridsen, during whose reign she was commonly styled dronning (“queen”) in Denmark, though she was never queen regnant nor a king's consort. Through Estrid, Sweyn traced his claim to the Danish throne and founded the matronymic House of Estridsen, which ruled from 1047 to 1412.

Medieval sources diverge on several points of her life. She was a daughter of Sweyn Forkbeard, but her mother is variously identified in sources as Sigrid the Haughty or, in earlier scholarship, as the Slavic princess sometimes called Gunhild of Wenden, a view now generally rejected. After Sweyn's death in 1014, Estrid came under Cnut's guardianship. He married her to a son of Grand Prince Vladimir the Great or Yaroslav the Wise, but the prince died shortly afterwards. Several western chroniclers, including Rodulfus Glaber and Adam of Bremen, record a proposed or short-lived marriage with the duke of Normandy (probably Robert I, Duke of Normandy), though the precise details remain uncertain. By the early 1020s, she was married to the English magnate Ulf Thorgilsson, who later served as regent in Denmark. By marriage she was thus, successively, a Russian princess, duchess of Normandy and a Danish noblewoman.

The House of Estridsen was a dynasty that provided the kings of Denmark from 1047 to 1412. The dynasty is named after its ancestor Estrid Svendsdatter. The dynasty is sometimes called the Ulfinger, after Estrid's husband, Ulf Jarl. The dynasty also provided three medieval rulers of Sweden and one of Norway. Their family coat of arms became the coat of arms of Denmark and thereby influenced the coat of arms of Tallinn and the coat of arms of Estonia.

The Royal Court of Denmark does not differentiate between different royal houses among the early Danish kings, but uses the term "the descent of Gorm the Old" about all the pre-Oldenburg monarchs.