Ronald Keith Siegel (born 1943) is an American psychopharmacologist who was an associate research professor in the Department of Psychiatry and Biobehavioral Sciences at the University of California, Los Angeles. Siegel is the author of several noted studies and books on psychopharmacology, hallucination, and paranoia. He has studied, lectured, and conducted research at Brandeis University, Harvard Medical School, Dalhousie University, and the Albert Einstein College of Medicine, and has been a consultant to several government commissions on drug use. His research has focused on the effects of drugs on human behavior, and has included numerous clinical studies in which human volunteers (sometimes referred to by Siegel as "psychonauts") have taken drugs such as ketamine, LSD, marijuana, mescaline, psilocybin, and THC.
In 2005, Siegel was an expert witness for the defense in the Robert Blake murder trial, testifying on the long-term effects of methamphetamine and cocaine use. According to the jury foreman in the trial, Siegel was "one of the most compelling witnesses" in discrediting the testimony of Ronald Hambleton, who claimed that Blake had asked him to murder Bonnie Lee Bakley. In the course of his testimony in the Blake trial, Siegel disclosed that in one study, he had taught monkeys to smoke crack cocaine.