Waldemar I, Prince of Anhalt-Zerbst (died 7 January 1368) was a German prince of the House of Ascania and ruler of the principality of Anhalt-Zerbst.
He was the youngest son of Albert I, Prince of Anhalt-Zerbst, by his second wife Agnes, daughter of Conrad, Margrave of Brandenburg-Stendal.
After the death of his father in 1316, the young Waldemar and his older brother Albert II were put under the custody of their maternal uncle Waldemar, Margrave of Brandenburg-Stendal. When both brothers reached adulthood, they ruled the principality of Anhalt-Zerbst jointly. Waldemar made his residence in Dessau and Albert in Zerbst or Köthen.
In 1359 Waldemar's nephew Albert III was also made co-ruler by his father Albert II, but he soon died. Three years later (in 1362), Albert II died and Waldemar became co-ruler with his youngest nephew John II.
Six years later, Waldemar died, leaving his only son Waldemar II as the new co-ruler with John II.
On 22 June 1344 Waldemar married Elisabeth (d. aft. 30 May 1351), daughter of Rudolf I, Elector of Saxony, and Duke of Saxe-Wittemberg. Her paternal grandmother Agnes of Habsburg was a daughter of Rudolph I, Holy Roman Emperor. The spouses were distantly related: Elisabeth's great-grandfather Albert I, Duke of Saxony, was a brother of Henry I, Count of Anhalt, Waldemar's great-great-grandfather. They had six children:
In 1365 Waldemar married for a second time to Beatrice (d. 1387), legitimized daughter of Obizzo III d'Este, Lord of Ferrara and Modena by his second wife and former mistress Lippa Ariosto. This union was childless.