Gertrud von Plettenberg (15??-26 October 1608), was administrator of several castles of the Electorate of Cologne and royal mistress of Ernest of Bavaria, Prince-Elector-Archbishop of Cologne. The couple may have married morganatically in secret around 1605.
Gertrud von Plettenberg was a member of the House of Plettenberg. Her family was poor and she was given a position at the court of Ernest of Bavaria in Arnsberg, Hirschberg and Höllinghofen. Because of her, Ernest primarily resided at Arnsberg from 1595 onward. Gertrud and Ernest were lovers from 1595, and reportedly married secretly in 1605. She had one son and one daughter with Ernest. From 1598, she is mentioned by contemporaries as his hostess. There were rumors at her death that she had been poisoned by the duchess of Cleves.