A Study in Scarlet is a detective novel by British author Arthur Conan Doyle. Published in 1887, the story marks the first appearance of Sherlock Holmes and Dr. Watson, who would go on to become one of the most well-known detective duos in literature.

The book's title derives from a speech given by Holmes, a consulting detective, to his friend and chronicler Watson on the nature of his work, in which he describes the story's murder investigation as his "study in scarlet": "There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it."

The story, and its main characters, attracted little public interest when it first appeared. Eleven complete copies of the magazine in which the story first appeared, Beeton's Christmas Annual for 1887, are known to exist now, which have considerable value. Although Conan Doyle wrote 56 short stories featuring Holmes, A Study in Scarlet is the first of only four full-length novels in the original canon. It was followed by The Sign of the Four, published in 1890.

A Study in Scarlet was the first work of detective fiction to incorporate the magnifying glass as an investigative tool.

In 1881, Doctor John Watson returns to London after serving in the Second Anglo-Afghan War and starts looking for a place to live. An old friend of his named Stamford tells him that Sherlock Holmes is looking for someone to split the rent at a flat at 221B Baker Street but cautions Watson about Holmes' eccentricities. Holmes and Watson meet and, after assessing each other and the rooms, they move in. Holmes is a "consulting detective", and his frequent guests are clients. After a demonstration of Holmes' deductive skills, Watson's disbelief turns into astonishment. A telegram requests a consultation in a murder case. Watson accompanies Holmes to the crime scene, an abandoned house on Brixton Road. Inspectors Gregson and Lestrade are already on the scene. The victim is Enoch Drebber of Cleveland, Ohio, and documents found on his person reveal that he has a secretary, Joseph Stangerson. On one wall, written in red, is "RACHE" (German for "revenge"), which Holmes dismisses as a ploy to fool the police. He deduces that the victim died from poison and supplies a description of the murderer. Upon moving Drebber's body, they discover a woman's gold wedding ring.

Holmes places notices in several newspapers about the ring and buys a facsimile of it, hoping to draw the murderer – who has apparently already tried to retrieve the ring – out of hiding. An old woman answers the advertisement, claiming that the ring belongs to her daughter. Holmes gives her the duplicate and follows her, but she evades him in a way that leads Holmes to believe that "she" was actually a young man in disguise, an accomplice to the murderer. A day later, Gregson visits Holmes and Watson, telling them that he has arrested a suspect. He went to Madame Charpentier's Boarding House where Drebber and Stangerson stayed before the murder. Drebber, a drunk, attempted to kiss Mrs. Charpentier's daughter, Alice, which caused their immediate eviction. He, however, came back later that night and attempted to grab Alice, prompting her older brother to attack him. He attempted to chase Drebber with a cudgel but claimed to have lost sight of him. Gregson has him in custody on this circumstantial evidence.

Stangerson is later murdered. His body is found near his hotel window, stabbed through the heart; above it was written "RACHE". The only things Stangerson had with him were a novel, a pipe, a telegram saying "J.H. is in Europe", and a small box containing two pills. Holmes tests the pills on an old and sickly Scottish Terrier in residence at Baker Street. The first pill produces no evident effect, but the second kills the terrier. A young street urchin named Wiggins then arrives. He is the leader of the Baker Street Irregulars, a group of street children Holmes employs to help him occasionally. Wiggins has summoned a cab Holmes wanted. Holmes sends him down to fetch the cabby, claiming to need help with his luggage. When the cabby comes upstairs, Holmes handcuffs and restrains him. The captive cabby is Jefferson Hope, the murderer of Drebber and Stangerson.

In Utah's Salt Lake Valley in 1847, John Ferrier and a little girl named Lucy, the only survivors of a small party of pioneers, are rescued from death by a party of Latter-day Saints led by Brigham Young, but only on the condition that they adopt and live under the Mormon faith. Years later in 1860, a now-grown Lucy befriends and falls in love with Jefferson Hope. However, Young forbids her from marrying outside the faith and demands that she marry either Joseph Stangerson or Enoch Drebber, both sons of members of the church's Council of Four. Ferrier, who has adopted Lucy and sworn never to marry his daughter to a Mormon, sends word to Hope in order to help them escape.