The Zodiac Killer is the pseudonym of an unidentified serial killer who operated in Northern California in the late 1960s. The case has been described as the most famous unsolved murder case in American history, becoming a fixture of popular culture and inspiring amateur detectives to attempt to resolve it.

The Zodiac murdered five known victims in the San Francisco Bay Area between December 1968 and October 1969, operating in rural, urban and suburban settings. He targeted young couples and a lone male cab driver, and his known attacks took place in Benicia, Vallejo, unincorporated Napa County, and the city of San Francisco proper. Two of his attempted victims survived. The Zodiac himself claimed to have murdered 37 victims, and he has been linked to several other cold cases, some in Southern California or outside the state.

The Zodiac originated the name himself in a series of taunting letters and cards that he mailed to regional newspapers, threatening killing sprees and bombings if they were not printed. Some of the letters included cryptograms, or ciphers, in which the killer claimed that he was collecting his victims as slaves for the afterlife. Of the four ciphers he produced, two remain unsolved, and one took 51 years to crack. While many theories regarding the identity of the killer have been suggested, the only suspect authorities ever publicly named was Arthur Leigh Allen, a former elementary school teacher and convicted sex offender who died in 1992.

Although the Zodiac ceased written communications around 1974, the unusual nature of the case led to international interest that has sustained throughout the years. The San Francisco Police Department marked the case "inactive" in April 2004, but re-opened it at some point prior to March 2007. The case also remains open in the city of Vallejo, as well as in Napa County and Solano County. The California Department of Justice has maintained an open case file on the Zodiac murders since 1969.