Magnus Ernst or Ernst Magnus Dönhoff (or Denhoff) (1581 - 1642 Elbląg) was a Baltic German who served the Polish-Lithuanian Commonwealth in Livonia and the Duchy of Prussia.
Always referred to as Magnus Ernst by Marion Gräfin Dönhoff, who wrote an extensive genealogy and history study of all the family branches, Magnus Ernst Dönhoff was the founder of the eastern Prussian line of the Dönhoff family. He came from Livonia and for services to George William, Elector of Brandenburg, he established his residence in Waldau near Königsberg. He was the brother of Gerhard Dönhoff and Kaspar von Dönhoff. He was married to Countess Katharina zu Dohna; they had four children. Three of their sons were born at Waldau.
Dönhoff was Castellan of Pärnu (Parnawa) from 1635, voivode of Parnawa from 1640, starost of Tartu (Derpsk), Telšiai (Telszew) and of Waldau in the Pregel valley in the Duchy of Prussia.
Dönhoff belonged to the commission which from 1625 worked for a peace agreement between Sweden and Poland. He was also instrumental in the negotiations at the peace of Altmark and at Sztumska Wieś. Dönhoff became Count (Reichsgraf) of the Holy Roman Empire since 1633. His son Friedrich Graf Dönhoff, born in Waldau in 1639, took up service for the Brandenburg electoral court.