John Ker, 3rd Duke of Roxburghe, KG, KT, PC (23 April 1740 – 1804) was a Scottish peer, scholar and publisher.

Born in Hanover Square, London, on 23 April 1740, Ker succeeded his father to become the 3rd Duke of Roxburghe in 1755.

In Jonathan Strange and Mr. Norrell, a 2004 novel by British writer Susanna Clarke, the Duke's affection for the Queen's sister and their subsequent separation are mentioned, and made a premise for an important aspect of the story.

One of the title characters, Gilbert Norrell, had a longstanding wish to examine the books of the Duke's library, believing there to be magical texts within.

The Duke's library contained several extremely rare and valuable tomes, which Mr. Norrell purchases at auction, causing an increase in friction between himself and Jonathan Strange.