Karin Nilsdotter (c. 1551–1613), was the royal mistress of Charles IX of Sweden, between 1568 and 1578.
Karin Nilsdotter was the daughter of Nicolaus Andrae (Nils Andersson), vicar of Östra Husby in Östergötland, and his spouse Karin.
In 1573, she is confirmed as such, and she was by then officially the chamber lady of the noblewoman Sigrid Kyle, spouse of Johan Olofsson till Sjösa, governor of Nyköping Castle, at Sjösa Manor.
The relationship was terminated before Charles' marriage in 1578, when she was awarded the Sundby Manor and married to Charles' courtier Gustav Andersson (d. 1584), with whom she had two daughters.
She remarried the personal physician of queen dowager Catherine Stenbock, and in 1594 she married thirdly to the nobleman Peder Kristensson Siöblad of Flättna, governor of Nyköping Castle, with whom she had a daughter.