Timothy Francis Leary (October 22, 1920 – May 31, 1996) was an American psychologist and writer, known for advocating psychedelic drugs. During American legality of LSD and psilocybin, Leary conducted experiments under the Harvard Psilocybin Project, resulting in the Concord Prison Experiment and the Marsh Chapel Experiment. Though obtaining useful data, Leary and his associate Richard Alpert were fired by Harvard University amid controversy surrounding such drugs.

Leary believed LSD showed therapeutic potential for use in psychiatry. He popularized catchphrases that promoted his philosophy such as "turn on, tune in, drop out" (a phrase given to Leary by Marshall McLuhan); "set and setting"; and "think for yourself and question authority". He also wrote and spoke frequently about transhumanist concepts involving space migration, intelligence increase and life extension (SMI²LE), and developed the eight-circuit model of consciousness in his book Exo-Psychology (1977).