Margaret Fernseed (c. 1560 – 28 February 1608) was an English prostitute, brothel keeper and murderer.
According to a pamphlet of 1608, Fernseed confessed to having run a brothel. The pamphlet details the methods by which she recruited prostitutes. Fernseed would "make spoile of young maidens who were sent out of the countrie by their friends with hope to advance themselves". Those girls were then put on the streets and compelled to hand over to Fernseed 10 shillings from what they earned. Married women who were discontented with their marriage were also targeted by Fernseed: once recruited, they were blackmailed by her if they refused to sleep with customers.
The pamphlet tells that Fernseed's husband, Anthony, was found dead in Peckham Fields near Lambeth, and that Fernseed had previously attempted to poison him. No motive is recorded, and Fernseed is documented as having declared her innocence of the murder.
Fernseed was executed for the murder of her husband on 28 February 1608.