The A.B.C. Murders is a work of detective fiction by British writer Agatha Christie, featuring her characters Hercule Poirot, Arthur Hastings and Chief Inspector Japp, as they contend with a series of killings by a mysterious murderer known only as "A.B.C.". The book was first published in the UK by the Collins Crime Club on 6 January 1936, sold for seven shillings and sixpence (7/6) while a US edition, published by Dodd, Mead and Company on 14 February of the same year, was priced $2.00.

The form of the novel is unusual, combining first-person narrative and third-person narrative. This approach was previously used by Agatha Christie in The Man in the Brown Suit. In The A.B.C. Murders the third-person narrative is supposedly reconstructed by the first-person narrator of the story, Arthur Hastings.

The initial premise is that a serial killer is murdering people with alliterative names. The murders follow an alphabetical order, starting with a victim whose initials were A. A., and appear to lack a motive.

The novel was well received in the UK and the US when it was published. One reviewer said it was "a baffler of the first water", while another remarked on Christie's ingenuity in the plot. A reviewer in 1990 said it was "a classic, still fresh story, beautifully worked out".

Returning from South America, Arthur Hastings meets with his old friend, Hercule Poirot, at his new flat in London. Poirot shows him a mysterious letter he has received, signed "A.B.C.", that details a crime that is to be committed very soon, which he suspects will be a murder. Two more letters of the same nature soon arrive at his flat, each before a murder being carried out by A.B.C., and committed in alphabetical order: Alice Ascher, killed in her tobacco shop in Andover; Elizabeth "Betty" Barnard, a flirty waitress killed on the beach at Bexhill; and Sir Carmichael Clarke, a wealthy man, killed at his home in Churston. In each murder, an ABC Rail Guide is left beside the victim.

The police team for the investigation, led by Chief Inspector Japp, includes Inspector Crome, who doubts Poirot's detective abilities, and Dr. Thompson, who tries to profile the killer. Poirot forms a "Legion" of relatives of the deceased to uncover new information: Franklin Clarke, Sir Carmichael's brother; Mary Drower, Ascher's niece; Donald Fraser, Betty's fiancé; Megan Barnard, Betty's elder sister, and Thora Grey, Sir Carmichael's young assistant. Following a meeting with the third victim's widow, Lady Clarke, one key similarity between the murders is established – on the day of each murder, a man selling silk stockings has appeared at or near each crime scene. Despite this information, Poirot wonders why the letters were sent to him, rather than the police or the newspapers, and why the third letter misspelled Poirot's address, causing a delay in his receipt of it. Soon, A.B.C. sends his next letter, directing everybody to Doncaster, where it is suspected that the next murder will occur at the St. Leger Stakes race meeting that day. However, the murderer strikes at a cinema instead, and the victim's name does not match the alphabetical pattern of the other killings.

The police soon get a tip-off about the man linked to the murders – Alexander Bonaparte Cust, an epileptic travelling salesman, who suffers from memory blackouts and constant agonising headaches as the result of a head injury during the First World War. Cust flees his apartment but collapses upon arriving at the Andover police station, where he is taken into custody. Apart from claiming that a stocking firm hired him, he lacks any memory of committing the murders but believes he must be guilty of them – he had been at the cinema when the last murder occurred and found blood on his sleeve and a knife in his pocket after he had left. The police learn that the firm in question never hired Cust. Their search of his room turns up an unopened box of ABC railway guides and the typewriter and fine paper used in A.B.C.'s letters, while the knife is discovered in the hallway outside his room where he dropped it. Poirot doubts Cust's guilt because of his memory blackouts, and especially because he had a solid alibi for the Bexhill murder.

Calling a Legion meeting, Poirot proclaims Cust's innocence, and exposes Franklin Clarke as the A.B.C. murderer. His motive was simple: Lady Clarke is slowly dying from cancer, and Sir Carmichael has left her a life interest in his estate, which would then go to his brother after her death. However, had Carmichael survived her, he would probably have married his assistant Thora and had children, who would have inherited instead. Franklin feared a possible second marriage, as he wants all of his brother's wealth, so he chose to murder his brother while Lady Clarke was still alive. A chance encounter with Cust at a pub gave Franklin the idea for the murder plot – he would disguise his crime as part of a serial killing. Having created the letters Poirot would receive, Franklin set up Cust with his job, giving him the typewriter and other items Franklin would use to frame him for the murders. A suggestion by Hastings makes clear that the third letter was misaddressed intentionally because Franklin wanted no chance of the police interrupting that murder. Franklin then followed Cust to the cinema, committed the last murder, and planted the knife on him as he left.