Ernst Magnus von Dönhoff (Polish: Ernest Magnus Denhoff; 1581–1642) was a Baltic German who served the Polish–Lithuanian Commonwealth in Livonia and the Polish fief of Duchy of Prussia.

He came from Livonia and for services to George William, Elector of Brandenburg, he established his residence in Waldau near Königsberg.

Dönhoff was castellan of Pärnu (Parnawa) from 1635, voivode of Parnawa from 1640, starost of Tartu (Derpsk), Telšiai (Telszew) and of Waldau in the Pregolya River valley in Duchy of Prussia.

Dönhoff belonged to the commission which from 1625 worked for a peace agreement between Sweden and Poland.

His son Friedrich Graf Dönhoff, born in Waldau in 1639, took up service for the Brandenburg electoral court.