Gilles Grenier is a fictional sorcerer created by Clark Ashton Smith for his short story, "The Mandrakes".
In the story, Gilles Grenier and his wife Sabine are 15th century sorcerers who move to the French province of Averoigne, where they set up a shop selling love potions made from the roots of mandrakes. Although these potions were their most popular product by far, they also dealt in charms and divination, and Gilles in particular was notable for his ability to "read infallibly the dictates of the stars". He was rumoured to come from a family of werewolves, and his hands and face were singularly hirsute, with a beard that grew almost to his eyes.
H. P. Lovecraft established Gilles Grenier as a descendant of Nush the Eternal, and an ancestor of both Hippolyte Le Sorcier and Smith in his family tree of Azathoth. However, Lovecraft labels Grenier as "Lord of Averoigne", which suggests that this might be a different character. The Gilles Grenier from Smith's story never had such power over Averoigne, being executed for the murder of his wife Sabine five years after arriving at the province.