Ribe Cathedral or Our Lady Maria Cathedral (Danish: Ribe Domkirke or Vor Frue Maria Domkirke) is located in the ancient city of Ribe, on the west coast of southern Jutland, Denmark. It was founded in the Viking Age as the first Christian church in Denmark by Ansgar, a missionary monk from Hamburg, under permission of the pagan King Horik I. The cathedral has experienced several damaging events throughout its long history and has been restored, expanded and decorated repeatedly. As it stands today, Ribe Cathedral is the best preserved Romanesque building in Denmark, but reflects a plethora of different architectural styles and artistic traditions. It ranks among the most popular tourist attractions in Denmark and has been awarded two stars in the Michelin guide.

Ribe is Denmark's oldest surviving city. Ribe began as an open trading market on the north bank of the Ribe River where it runs into the North Sea. Danes, Norwegians, Swedes, Germans, Frisians, English and other cultures occasionally brought exchange goods here from all parts of northwestern Europe. The landscape around Ribe is flat, wind-blown and sandy, without any particular natural harbour, but in former times, boats could enter the river from the sea and reach the town. By the late Medieval period, the natural sedimentation processes of the Wadden Sea had closed that option. Ribe Market was sanctioned by either King Angantyr (Ongendus) or King Harald Hildetand as early as 705. In the Viking Age, Ribe was a bustling international trade center, making it an ideal starting point for Christian missionaries from Hamburg to initiate the process of the Christianization of Scandinavia.

Harald Klak, king in Jutland, was forced in to exile by his King, Horik I and fled to Germany to get help from Louis the Pious, King of the Franks. Louis the Pious put off Harald's request and offered him the Dukedom of Frisia as a consolation prize, if he would become Christian. Harald agreed and was baptized along with his wife, family and "four hundred Danes" in his company. Harald returned to Denmark in 826 in an attempt to reclaim his former lands and brought the missionary monk Ansgar with him in order to continue the Christianization of the Danes. Harald's quarrelsome nature soon asserted itself and he fled back to Frisia and Ansgar was forced to leave Denmark.

The first church in Ribe was founded in 860 by Ansgar, who later became Archbishop of Hamburg. It was a timber church, built with the permission of King Horik I on the south side of the river, across the market. Ansgar won the confidence of the king, who had burned Hamburg in 845, Ansgar's home town. But the Danes in Hedeby and Ribe soon complained about the church bells, which they feared might scare away the land sprites (Danish: landvætter). So in reaction to the increased success of Ansgar and his companions, Young King Horik II closed the church in Hedeby. Eventually Ansgar also won Horik the Younger's friendship, and the church was later reopened.

St. Rimbert (ca. 830–888) who succeeded Ansgar, died in 888. Following his death, the mission to the Danes collapsed, the missionaries moved on, and the church was burned down to eradicate any memory of the "foreign" religion.

Ribe was established as a diocese for St. Leofdag, the first Bishop of Ribe, in 948 under the supervision of the Archbishop of Hamburg, who was authorized by the Roman Curia to proselytize the Danes. Leofdag was murdered that same year, when a housecarl skewered him with a spear, as he forded the river at Ribe. Although never canonized, Leofdag was revered as a local saint, until the Reformation. His remains would eventually end up in Ribe Cathedral.

The first stone cathedral, was begun by Bishop Thur in 1110 and stood completed in 1134. Tufa stone was imported from Germany to build the permanent structure, as stone was a scarce resource around Ribe. The cathedral was built in the Romanesque style, with half-rounded arches supporting a flat timber ceiling, a typical basilica style building, patterned after churches in northern Germany.

King Erik II of Denmark (Danish:Erik Emune) was murdered at the Urnehoved Assembly (Danish: Ting) in July 1137 by Chief Sorte Plov in revenge for the execution of a relative. The king was buried inside the cathedral. King Erik's three-year reign, was a short pause in the unrelenting competition for the throne of Denmark. King Erik earned the nickname "Bloody Erik" for his execution of his own brother Harald and ten (or eleven) of Harald's twelve sons. Later historians have nevertheless called him "Erik Who ought to be Remembered" (Danish: Emune) in their writings. The royal burial enhanced the Cathedral's reputation.