Wilhelm Fliess (German: Wilhelm Fließ; 24 October 1858 – 13 October 1928) was a German Jewish otolaryngologist who practised in Berlin.
He developed highly eccentric theories of human biorhythms and a possible nasogenital connection that have not been accepted by modern scientists.
He is today best remembered for his close personal friendship and theoretical collaboration with Sigmund Freud, a controversial chapter in the history of psychoanalysis.