| Valdemar IV | |
| Succession: | Duke of Schleswig |
| Tenure: | 1283 - 1312 |
| Predecessor: | Eric I |
| Successor: | Eric II |
| Birth Date: | c. 1262 |
| Death Date: | 1312 |
| Noble Family: | Estridsen |
| Spouse: | Elisabeth of Saxe-Lauenburg Anastasia of Schwerin |
| Issue: | Eric II, Duke of Schleswig |
| Father: | Eric I, Duke of Schleswig |
| Mother: | Margaret of Rugia |
Valdemar IV Eriksøn (born c. 1262, died 1312) was Duke of Schleswig from 1283 until his death in 1312. He was the eldest son of Duke Eric I of Schleswig and Margaret of Rugia.
At the death of his father Duke Eric I in 1272, Valdemar was only about 10 years old. For the following ten years, King Eric V of Denmark, feudal overlord over the Duchy of Schleswig, refused to install Valdemar in his father's duchy. Only in 1283, at a meeting of the Danehof in the city of Vordingborg, did the Danish magnates force King Eric to install Valdemar as duke. In 1284 he granted city rights to the city of Flensburg.
However, Valdemar was not content and also claimed the island of Als, as well as the Crown lands of Schleswig. When in 1284 a judgment of the Danehof ruled against the claims of Duke Valdemar, he occupied the island of Als by force. However, the military campaign proved disadvantageous for the duke, and as he intended to travel north to gain support in Sweden and Norway, he was captured by King Eric and confined at Søborg Castle in 1285. He was released the following year on condition that he abandoned almost all of his claims.
Just half a year later, in November 1286, King Eric was assassinated, and Duke Valdemar naturally fell under suspicion. The Queen Dowager, Agnes of Brandenburg, apparently shared the suspicion, but feared open aggression with the Duke. Instead, his support was achieved by accepting his claims and appointing him regent of the kingdom during the minority of King Eric's under age son and successor, Eric VI.
Already in 1289, the confidence between Duke Valdemar and Dowager Queen Agnes was breached and a clash occurred between the Duke and the followers of the Queen. Shortly after, Duke Valdemar even entered into a league with King Eric II of Norway, then at war with Denmark. Apparently, in 1295 Valdemar suffered a defeat at a naval battle in the channel of Grønsund. In 1297, a final settlement was concluded where Duke Valdemar among other things abandoned his claims to the disputed island of Als.
Minor disputes with King Eric VI continued during the following years. On a whole, however, the Duke was hence on friendly terms with the King and supported him in his wars in Sweden and the Holy Roman Empire. In 1311 he participated in the magnificent tournament held by King Eric in Rostock.
Duke Valdemar died in the spring of 1312. He was succeeded as Duke by his son Eric II who had already been his father's coruler for several years. Valdemar was buried in the Schleswig Cathedral.
In 1287 Duke Valdemar married Elisabeth of Saxe-Lauenburg, a daughter of John I, Duke of Saxony. This marriage produced the only legitimate son of Duke Valdemar:
In 1306 he married Anastasia of Schwerin, a daughter of Niklot I, Count of Schwerin.