Guillaume Grandidier (1 July 1873 – 13 September 1957) was a French geographer, ethnologist, and zoologist who studied the island of Madagascar.

He was the son of the wealthy industrialist Alfred Grandidier also a zoologist and expert on Madagascar.

Guillaume Grandidier was Secretary of the Geographical Society of Paris and a prolific author.

This work was undertaken in cooperation with his father and others such as Alphonse Milne-Edwards and Leon Vaillant.

Liopholidophis grandidieri, a species of snake endemic to Madagascar, was named in his honor by French herpetologist François Mocquard.