The Zodiac Killer is the pseudonym of an unidentified serial killer who murdered five known victims in the San Francisco Bay Area between December 1968 and October 1969. The case has been described as "arguably the most famous unsolved murder case in American history," and has become both a fixture of popular culture and a focus for efforts by amateur detectives.

The Zodiac's known attacks took place in Benicia, Vallejo, unincorporated Napa County, and the City and County of San Francisco proper. He attacked three young couples and a lone male cab driver. Two of these victims survived. The Zodiac coined his name in a series of taunting messages that he mailed to regional newspapers, in which he threatened killing sprees and bombings if they were not printed. He also said that he was collecting his victims as slaves for the afterlife. He included four cryptograms or ciphers in his correspondence; two were decrypted in 1969 and 2020, and two are generally considered to be unsolved.

In 1974, the Zodiac claimed 37 victims in his last confirmed letter. This tally included victims in Southern California such as Cheri Jo Bates, who was murdered in Riverside in 1966. Despite many theories about the Zodiac's identity, the only suspect authorities ever named was Arthur Leigh Allen, a former elementary school teacher and convicted sex offender who died in 1992.

The unusual nature of the case led to international interest that has been sustained throughout the years. The San Francisco Police Department marked the case "inactive" in 2004 but re-opened it prior to 2007. The case also remains open in the California Department of Justice, Federal Bureau of Investigation, the city of Vallejo, as well as in Napa and Solano counties.

Investigators agree on four confirmed attacks by the Zodiac Killer in California. Five victims were killed during these attacks, and two survived:

From 1969 to 1974, the Zodiac mailed heavily misspelled letters and ciphers to law enforcement and media outlets. Some letters began, "This is the Zodiac speaking" and were signed with a symbol resembling the crosshairs of a gunsight: . Four of the mailings had a cryptogram enclosed. Two have been solved, in 1969 and 2020. The letters were postmarked in San Francisco and Pleasanton.

The Zodiac's confirmed correspondence with date, recipient, and incipit:

The first murders retroactively attributed to the Zodiac were the shootings of high school students Betty Lou Jensen (16) and David Arthur Faraday (17) on December 20, 1968. Faraday was a student at Vallejo High School, while Jensen was a student at Hogan High School. At 8:30 p.m. Faraday picked up Jensen, and the couple visited one of Jensen's friends. Sometime after 9 p.m., they drove to the outskirts of Vallejo and parked at a lover's lane on Lake Herman Road, just inside Benicia city limits. Between 10:15 and 10:30 p.m., a passing motorist noticed the couple parked on a gravel runoff near the gate to a water pumping station. They were spotted again at 11 p.m.