William Palmer (6 August 1824 – 14 June 1856), also known as the Rugeley Poisoner or the Prince of Poisoners, was an English doctor and murderer. Palmer was convicted for the 1855 murder of his friend John Cook, whom he had poisoned with strychnine. He was also suspected of poisoning several other people including his brother and his mother-in-law, as well as four of his children. Palmer made large sums of money from the deaths of his wife and brother after collecting on life insurance, and by defrauding his wealthy mother.

Palmer was hanged in public the year following his conviction. Charles Dickens called Palmer "the greatest villain that ever stood in the Old Bailey".

William Palmer was born in Rugeley, Staffordshire, the sixth of eight children of Joseph and Sarah Palmer. His father worked as a sawyer and died when William was aged 12, leaving Sarah with a legacy of £70,000.

At age 17, Palmer worked as an apprentice at a Liverpool chemist, but was dismissed after three months following allegations of theft. He studied medicine in London and qualified as a physician in August 1846. After returning to Staffordshire later that year, Palmer met plumber and glazier George Abley at the Lamb and Flag public house in Little Haywood, and challenged him to a drinking contest. Abley accepted, and an hour later was carried home, where he died in bed later that evening; nothing was ever proved, but locals noted that Palmer had an interest in Abley's attractive wife.

Palmer returned to Rugeley to practise as a doctor and, in St. Nicholas Church, Abbots Bromley, married Ann Thornton (born 1827; also known as Brookes as her mother was the mistress of a Colonel Brookes) on 7 October 1847. His mother-in-law, also called Ann Thornton, had inherited a fortune of £8,000 after Colonel Brookes committed suicide in 1834. The elder Thornton died on 18 January 1849, two weeks after coming to stay with Palmer; she was known to have lent him money. An elderly Dr. Bamford recorded a verdict of apoplexy. Palmer was disappointed with the inheritance he and his wife gained from the death, having expected it to be much greater.

Palmer became interested in horse racing and borrowed money from Leonard Bladen, a man he met at the races. Bladen lent him £600, but died at Palmer's house on 10 May 1850. Palmer's wife was surprised to find that Bladen died with little money on him despite having recently won a large sum at the races; his betting books were also missing, so there was no evidence of his having lent Palmer any money. Bladen's death certificate listed Palmer as "present at the death" and stated the cause of death as "injury of the hip joint, 5 or 6 months; abscess in the pelvis".

Palmer's first son, William Brookes Palmer, was born towards the end of 1848 and christened in January 1849. He outlived his father, dying on 29 April 1926. The Palmers had four more children, all of whom died in infancy. The cause of death for each child was listed as "convulsions":

As infant mortality was not uncommon at the time, these deaths were not initially seen as suspicious, though after Palmer's conviction in 1856 there was speculation that he had administered poison to the children to avoid the expense of more mouths to feed. By 1854 he was heavily in debt and resorting to forging his mother's signature to pay off creditors. Palmer took out life insurance on his wife with the Prince of Wales Insurance Company, and paid out a premium of £750 for a policy of £13,000. Her death followed on 29 September 1854, at twenty-seven years old. She was believed to have died of cholera, as a cholera pandemic was affecting the United Kingdom (causing 23,000 deaths across the country) at the time.