Kenneth Alessio Bianchi (/ˈbiːænkiː/; born May 22, 1951) is an American serial killer, kidnapper, and rapist. He is known for the Hillside Strangler murders which he committed with his cousin Angelo Buono Jr. in Los Angeles, as well as for two more murders in Washington State as the sole perpetrator. Bianchi is currently serving a sentence of life imprisonment in Washington State Penitentiary for these crimes. He was also at one time a suspect in the Alphabet murders, three unsolved murders in his home city of Rochester, New York, from 1971 to 1973. Bianchi was most recently denied parole in 2025.

Kenneth Bianchi was born on May 22, 1951, in Rochester, New York, to a 17-year-old prostitute who gave him up for adoption two weeks after he was born. He was adopted in August 1951 by Italian-American couple Nicholas Bianchi and his wife, Frances Scioliono-Bianchi, and was their only child. Bianchi was deeply troubled from a young age, with his adoptive mother describing him as "a compulsive liar" from the time he could talk. He would often fall into inattentive, trance-like daydreams where his eyes would roll back into his head. From these symptoms, a physician diagnosed the five-year-old Bianchi with petit mal seizures. He was also frequently given physical examinations by doctors due to a problem with involuntary urination, causing him a great deal of humiliation.

Bianchi was prone to fits of anger as well as bouts of insomnia and habitual bedwetting when he was young. On January 2, 1957, he accidentally fell off a jungle gym and landed on his face. Bianchi's adoptive mother, in an attempt to change his behavior, sent him to a private Catholic elementary school and to sessions with a psychiatrist, with Bianchi being diagnosed with a passive-aggressive personality disorder at the age of ten. Bianchi's intelligence quotient was measured at 116 at the age of eleven, but despite having above-average intelligence, he was an underachiever and was removed twice from schools after failing to get along with teachers. Bianchi's adoptive mother described him as "lazy," and his teachers claimed that he was working below his potential.

In July 1963, Bianchi pulled down a six-year-old girl's underwear after deciding that he liked doing it. After his adoptive father died suddenly from pneumonia in 1964, the teenaged Bianchi refused to cry or show any other signs of grief. After her husband's death, Bianchi's adoptive mother had to work while he attended a public high school and was known for keeping Bianchi home from school for long periods of time. Nonetheless, Bianchi dated frequently and even joined a motorcycle club. Shortly after he graduated from Gates-Chili High School in 1970, Bianchi married his high school sweetheart, Brenda Beck. The union ended after eight months. Supposedly, she left him without an explanation.

As an adult, Bianchi decided that he wanted to become a police officer, and he enrolled at Monroe Community College to study police science and psychology. He dropped out of college after just one semester, however, and drifted through a series of menial jobs, finally finding employment as a security guard at a jewelry store. This gave Bianchi the opportunity to steal valuables, which he often gave to girlfriends or prostitutes to buy their loyalty. He applied for a position at the local sheriff's department but was rejected.

Because of his many petty thefts, Bianchi was constantly on the move. In 1976 he moved to Los Angeles, and started spending time with his older cousin and Frances' nephew, Angelo Buono Jr., who impressed Bianchi with his fancy clothes, jewelry and talent for getting any woman he wanted and "putting them in their place." Before long, they worked together as pimps and, by late 1977, had escalated to committing what would become known as the "Hillside Strangler" murders. Bianchi and Buono had raped and murdered ten young women and girls by the time they were arrested in early 1979.

Bianchi and Buono frequently impersonated police officers, flashing phony police badges and ordering the unassuming victims to get into Bianchi's Cadillac, telling them that it was an unmarked police car. They'd then handcuff them and drive them to Buono's upholstery shop, where they would proceed to sexually abuse their victims before strangling them. They experimented with other methods of killing, such as lethal injection, electric shock and carbon monoxide poisoning. Even while committing the murders, Bianchi applied for a job with the Los Angeles Police Department (LAPD) and had even been taken for several ride-alongs with police officers while they were searching for the Hillside Strangler. The crimes and murders that the duo committed are as follows:

At his trial, Bianchi pleaded not guilty by reason of insanity, claiming that another personality, one "Steve Walker," had committed the crimes. It was believed he had recently seen the film Sybil (1976), about a woman suffering from multiple personalities triggered by childhood abuse. Bianchi convinced a few expert psychiatrists that he indeed suffered from multiple personality disorder, but investigators brought in their own psychiatrists, mainly Martin Orne. When Orne mentioned to Bianchi that in genuine cases of the disorder, there tend to be three or more personalities, Bianchi promptly created another alias, "Billy."