Théodore Flournoy (15 August 1854 – 5 November 1920) was a professor of psychology at the University of Geneva and author of books on parapsychology and spiritism.

He is best known for his study of the medium Helen Smith (or Hélène Smith - a pseudonym for Catherine Muller) who relayed information about past lives through a trance state, entitled From India To The Planet Mars (1899). Flournoy described her outpourings as the products of cryptamnesia and as 'romances of the subliminal imagination,' - as evidence of the unconscious mind.

His book Spiritism and Psychology (1911) translated by Hereward Carrington claimed more broadly that mediumship could be explained by suggestion and telepathy from the medium's subconscious mind and that there was no evidence for the spirit hypothesis.