Valdemar Knudsen (also Waldemar, born 1157/1158; died April 1235 or 1236 probably in Cîteaux) was a prince of Denmark. His mother gave birth to him as the posthumous illigitimate son of Canute V of Denmark. Valdemar officiated as Bishop of Schleswig (1182–1208), Steward of the Duchy of Schleswig (1182–1193), and Prince-Archbishop of Bremen (1192/1206–1218).

His father, Canute V, was slain on 9 August 1157 by the co-regent Sweyn III. So Valdemar, like his half-brother, Saint Niels of Århus, claimed succession to the Danish throne.

Valdemar grew up at the court of his cousin Valdemar I of Denmark, the Great. Valdemar studied in Paris. After his studies his cousin promoted Valdemar's provision for the See in Sleswick (, ) in 1178/1182.

Valdemar as Bishop of Schleswig and Steward of the Duchy of Schleswig

In 1187/1188 Valdemar was consecrated probably in the Cathedral St. Petri. From 1184 on Valdemar officiated as steward of the Duchy of Schleswig for Valdemar the Great's minor son Duke Valdemar (later King Valdemar II of Denmark). In 1187 and 1188 Hartwig of Uthlede, Prince-Archbishop of Bremen, and his troops invaded the trans-Elbian free peasants republic of Ditmarsh, religiously belonging to the Archdiocese of Bremen, in order to subject Ditmarsh also to his secular princely overlordship. The free peasants promised to pay him dues, only to mock about him, once he and his soldiers had left. The Ditmarsians gained support by steward Valdemar, so that Hartwig could not dare another costly invasion.

Valdemar learned about the abbot, monks, and nuns at the Benedictine Abbey of St. Michael's in the town of Schleswig, who had fallen into immoral behavior and had earned a reputation for drunkenness. Valdemar decided that the best way to reform the monks was to move them from the temptations in town, and establish a new house far enough away that the monks wouldn't cause more trouble. In 1192 the monks apparently were moved unwillingly to Güldenholm to begin the work on a new monastery. At some point, Güldenholm Abbey became Cistercian, with the monks working in the fields to earn their daily bread and meat.

In 1190 Emperor Henry VI, House of Hohenstaufen, dismissed Hartwig of Uthlede as Prince-Archbishop of Bremen for his partisanship with the House of Guelph. In 1192 Duke Valdemar, meanwhile 22 years old, seriously disagreed about Bishop Valdemar's way of ruling the Duchy of Schleswig. In the same year the Bremian Chapter didn't wait any longer for a papal dismissal of Hartwig and unauthorisedly elected Valdemar as its new Prince-Archbishop - encouraged by Henry VI. Valdemar welcomed his election, hoping his new position could be helpful in his dispute with Duke Valdemar and his elder brother Canute VI. Before entering the Prince-Archbishopric he won the support of Ditmarsh. The Bremian mint in the city of Bremen issued coins showing Valdemar's portrait.Adolf Hofmeister, "Der Kampf um das Erbe des Stader Grafen zwischen den Welfen und der Bremer Kirche (1144–1236)", In: Geschichte des Landes zwischen Elbe und Weser: 3 vols., Hans-Eckhard Dannenberg and Heinz-Joachim Schulze (eds.) on behalf of the Landschaftsverband der ehemaligen Herzogtümer Bremen und Verden, Stade: Landschaftsverband der ehem. Herzogtümer Bremen und Verden, 1995 and 2008, (Schriftenreihe des Landschaftsverbandes der ehem. Herzogtümer Bremen und Verden; No. 7), vol. II: Mittelalter (1995), pp. 105–157, here p. 119. ISBN 978-3-9801919-8-2

Valdemar as canon-law Prince-Archbishop of Bremen in Danish captivity (1193–1206)

Duke Valdemar realised the threat Prince-Archbishop Valdemar presented. In 1192 he invited the Prince-Archbishop to meet him in Åbenrå. Then the bishop fled to Swedish Norway to avoid arrest. The following year he organised - supported by the Hohenstaufens - a fleet of 35 ships and harried the coasts of Denmark with an eye to overthrowing King Canute VI of Denmark, claiming the Danish throne for himself. In 1193 King Canute VI of Denmark captured him, before he could ever enter the Prince-Archbishopric of Bremen. Despite the interventions by the Holy See Valdemar stayed in captivity in Nordborg (1193–1198) and then in the tower at Søborg Castle on Zealand until 1206. So he in effect couldn't take the Bremian See. Valdemar was released upon the initiative of the Danish Queen Dagmar and Pope Innocent III and after swearing, never to interfere again in Danish affairs. Valdemar left Søborg for Rome.

Valdemar as canonically dismissed, but de facto ruling Prince-Archbishop of Bremen

When in 1207 Hartwig of Uthlede died, a majority of Bremian Capitulars - overlooking the votes of the absent constitutionally provided three representatives of Hamburg's subchapter - again elected Valdemar. A minority, led by the capitular provost Burchard, Count of Stumpenhusen fled to Hamburg. Valdemar, still asserting himself as Prince-Archbishop, could not hinder Yso of Wölpe, Prince-Bishop of Verden, to capture the Bremian castle in Ottersberg. Hamburg and the neighboured County of Holstein, both part of the diocesan, but not of the prince-archiepiscopal territory, were subject to Danish occupation under Valdemar II, who in 1202 had confederated himself with Otto IV, House of Guelf, rival king against the German King Philip. Philip, House of Hohenstaufen, recognised bishop Valdemar as the legitimate Prince-Archbishop of Bremen, because thus the Prince-Archbishopric would become his ally against Valdemar II.
Valdemar II and the fled capitulars protested at Pope Innocent III, who first wanted to research the case. When bishop Valdemar left Rome for Bremen against Innocent's order to wait his decision, he banned Valdemar by an anathema and in 1208 finally dismissed him too as bishop of Schleswig. In early 1208 the fled capitulars and the local Subchapter in Hamburg elected Burchard as rival prince-archbishop and Valdemar II, usurping imperial power, invested Burchard with the regalia - with effect only in the prince-archiepiscopal and diocesan territory north of the Elbe.

Prince-Archbishop Valdemar confederated with the free peasants of Stedingen, a region within the Prince-Archbishopric whose inhabitants rejected to be subjected as serfs. Earlier Burchard, then still provost of the Bremian Chapter, had failed to subject the free peasants by military means. This weakness provoked the neighboured Count Maurice of Oldenburg to subject them and annex Stedingen – also in vain. The free peasants of Stedingen agreed to provide Prince-Archbishop Valdemar with mercenaries, who in return retained any further attack on their freedom.

In 1208 Valdemar II invaded with Danish troops the prince-archiepiscopal territory south of the Elbe and conquered Stade. In August Prince-Archbishop Valdemar reconquered the city only to lose it soon after again to Valdemar II, who now built a bridge over the Elbe and fortified a forward post in Harburg upon Elbe.
In Bremen Prince-Archbishop Valdemar had been warmly welcomed and nobody cared about the anathema. After Philip's assassination in June 1208, Prince-Archbishop Valdemar as well as the burghers and the city of Bremen joined the party of the former rival king Otto IV, whom Innocent III crowned Emperor in 1209. Otto IV persuaded Valdemar II to withdraw into the north of the Elbe, urged Burchard to resign and expelled Prince-Archbishop Valdemar. Now Valdemar fled to Rome and craved Innocent's forgiving, and he pardoned him.

Bremen's capitular dean as well as its suffragans Albert of Bexhövede, Bishop of Livonia, and Prince-Bishop Dietrich I of Lübeck, proposed Burchard's uncle Gerhard I, Count of Oldenburg-Wildeshausen, already Prince-Bishop of Osnabrück, for the See. In 1210 Innocent III made him the new Prince-Archbishop. In 1211 Duke Bernard III of the younger Duchy of Saxony escorted his brother-in-law Valdemar, the papally dismissed Prince-Archbishop, into the city of Bremen, de facto regaining the See and enjoying the sudden support of Otto IV, who meanwhile fell out with Innocent over Sicily. As a reaction Valdemar II recaptured Stade, while in 1213 Otto's elder brother Henry V, Count Palatine of the Rhine, conquered it for Prince-Archbishop Valdemar. In 1215 Henry repelled another Danish attack on Stade.

In 1212–1214 the mercenaries from Stedingen destroyed the castles at Beverstedt, Stotel (a part of today's Loxstedt), Riensberg and Seehausen (both a part of today's city of Bremen), all held by partisans of Valdemar's rival Prince-Archbishop Gerhard I, Count of Oldenburg-Wildeshausen, whom the Stedingers clearly identified as a proponent of their subjection to serfdom. The Oldenburgers successfully defended the castle in Hagen im Bremischen against the Stedingers and Gerhard I mobilised the Count Henry I of Hoya to help, who inflicted the first defeat on the Stedingers in 1213.

But soon Otto IV's position was challenged by Frederick II, in 1215 replacing Otto as accepted Emperor. Nevertheless, in the same year Henry V, his younger brother Otto IV, Margrave Albert II of Brandenburg, and Prince-Archbishop Valdemar and their troops, among them mercenaries from Stedingen, conquered Hamburg. In the winter of 1216 Valdemar II and his Danish troops, unable to take the city of Stade, ravaged the County of Stade and reconquered Hamburg.

In 1216 the mercenaries of Stedingen swung over to the party of Gerhard I, who promised to respect their freedom, and attacked the city of Bremen, loyal to Valdemar. Henry V rescued the city with his troops. In 1217 the city of Bremen deserted Valdemar's party. Now Henry V, Otto IV and their troops ravaged the Prince-Archbishopric (so-called Valdemarian Turmoils, 1217–1218). In 1218 Gerhard I and Valdemar II allied to expell Henry and Otto from the Prince-Archbishopric. Gerhard's troops approached the fortress of Vörde disguised as sick, lining up for a treatment by the faith healer and farmer Otbert. Once arrived they overthrew Henry's soldiers in the fortress. After the deaths of Otto in 1218 and Gerhard I in 1219, Henry V reached an agreement with the new Prince-Archbishop Gerhard II, to maintain the County of Stade as prince-archiepiscopal vassal.

Valdemar as a monk

In 1218 Valdemar fled the Prince-Archbishopric towards his nephew Duke Albert I of the younger Duchy of Saxony. Later Valdemar entered the monastery of Loccum. In 1220 the anathema against him was lifted. In the same year or only in 1232 Valdemar moved into the monastery of Cîteaux, where he died in 1235 or 1236. He was buried on 28 April 1236 in the Loccum Abbey.