Jacques Lucien Monod (9 February 1910 – 31 May 1976) was a French biologist who was awarded a Nobel Prize in Physiology or Medicine in 1965, sharing it with François Jacob and Andre Lwoff "for their discoveries concerning genetic control of enzyme and virus synthesis".

Monod (along with François Jacob) is famous for his work on the E. coli lac operon, which encodes proteins necessary for the transport and breakdown of the sugar lactose (lac). From their own work and the work of others, he and Jacob came up with a model for how the levels of some proteins in a cell are controlled. In their model, the manufacture of proteins, such as the ones encoded within the lac (lactose) operon, is prevented when a repressor, encoded by a regulatory gene, binds to its operator, a specific site on the DNA next to the genes encoding the proteins. It is now known that repressor bound to the operator physically blocks RNA polymerase from binding to the promoter, the site where transcription of the adjacent genes begins.

Study of the control of expression of genes in the lac operon provided the first example of a transcriptional regulation system. He also suggested the existence of mRNA molecules that link the information encoded in DNA and proteins. Monod is widely regarded as one of the founders of molecular biology. Monod's interest in the lac operon originated from his doctoral dissertation, for which he studied the growth of bacteria in culture media containing two sugars.
Monod was also awarded several other honours and distinctions, among them the Légion d'honneur.