Ferdinand Julius Cohn (January 24, 1828 - June 25, 1898) was a German biologist.

Cohn was born in Breslau (Wrocław) in the Prussian Province of Silesia. At the age of 10 he suffered hearing impairment. He received a degree in botany in 1847 at the age of nineteen. Cohn was the first to classify bacteria as plants. He also divided bacteria into four groups: Sphaerobacteria, Microbacteria, Desmobacteria, and Spirobacteria. In 1885 he received the Leeuwenhoek Medal.

Cohn is also known for his description of the life cycle of Bacillus and for showing that it changes from a vegetative state to an endospore when subjected to a "hostile" environment (e.g., high heat). Therefore, sterilization by boiling is not effective. It will only kill the bacteria in the vegetative state and the endospores remain viable.

Cohn died in Breslau.