Alexandre Emile Jean Yersin (September 22, 1863 – 1 March 1943) is a Swiss and naturalized French physician and bacteriologist.

He is remembered as the co-discoverer of the bacillus responsible for the bubonic plague or pest, which is later named in his honor (Yersinia pestis).

Another bacteriologist, Kitasato Shibasaburō, is often credited with independently identifying the bacterium a few days earlier but may have identified a different bacterium and not the pathogen causing plague.

Yersin also demonstrates for the first time that the same bacillus is present in the rodent as well as in the human disease, thus underlining the possible means of transmission.