Claude Vivier (French: [klod 'vivje] VEEV-yay; baptised as Claude Roger; 14 April 1948 – 7 March 1983) was a Canadian composer, pianist, poet and ethnomusicologist of Québécois origin. After studying with Karlheinz Stockhausen in Cologne, Vivier became an innovative member of the "German Feedback" movement, a subset of what is now known as spectral music. Between 1976 and 1977, Vivier travelled to Egypt, Japan, Iran, Thailand, Singapore, and Bali, where he came under the influence of aspects of their respective traditional musics.

Despite working at a slow pace and leaving behind a small œuvre, Vivier's musical language is vast and diverse. His place in the spectral movement of Europe entailed the manipulation of the harmonic series and led to music that incorporated microtones to replicate these frequencies, a compositional technique he would later refer to as the jeux de couleurs. He is also known for incorporating elements of serialism, dodecaphony, musique concrète, extended techniques, surrealism, traditional Québécois folk songs, and more. The themes of Vivier's pieces are largely seen as autobiographical – often centring around loneliness and ostracization, the search for love and companionship, or voyaging to foreign lands. He used his personal experiences to advance an avant-garde style, having written multilingual vocal music and devising his so-called langues inventées (invented languages). He is considered to be among the greatest composers in Canada's history – György Ligeti would revere Vivier as "the most important and original composer of his generation."

Vivier was openly gay. After ending his brief relationship with Christopher Coe, he frequented Parisian gay bars where he solicited male prostitutes, one of whom violently attacked him in January 1983. Despite warnings from friends and his own increasing paranoia over his safety, Vivier had continued to engage in the same behaviour. On the night of 7 March, he was killed by a serial murderer who routinely deceived gay men in The Marais in order to rob and assault them.

Claude Vivier is believed to have been born on 14 April 1948 in the vicinity of Montréal, Québec, and was voluntarily placed in the orphanage of La Crèche Saint-Michel (no longer in operation) that same day by his mother. Her name, ethnicity, and origin, as well as that of Vivier's father, are unknown.

Vivier himself would posit in later years that he was likely not of French Canadian heritage. He would often mythicize the story and heritage of his parents, at times telling people his family was German, Eastern European, or Jewish. His friend Philippe Poloni would relay: "he thought that his father was a conductor, or his mother was a musician, and they met in Montréal. Or something like that, something very romantic. He always said he spoke good German and good Italian because he had a natural connection with those two languages as he had some Italian and some Jewish German blood in his veins". He searched his whole life in the hope of finding his birth parents, to no avail. This frustration and the feeling of a hollow identity inspired many of his works, including Lonely Child (1980).

After receiving the young boy, the Sœurs Grises who ran the orphanage gave him the name of Claude Roger and he was subsequently baptized at the Église Saint-Enfant-Jésus. He was considered mentally disabled, as the nuns believed him to be "deaf and dumb." Apart from this, however, very little is known from his early life in the orphanage due to a lack of record-keeping; any learning disability that he may have had went undiagnosed.

He was adopted at the age of two and a half by the working-class Vivier family from Mile End, consisting of parents Armand and Jeanne (née Masseau) and their two biological children. The couple had suffered a miscarriage many years prior and were looking for a young girl to adopt, only to find each of the Montréal orphanages having solely boys. It is unknown why Claude was chosen out of the many in Saint-Michel. He was a charismatic and excitable child, but his time in the large and strictly Catholic Vivier household was fraught with incidents. He is reported to have learned to speak at the age of six, before which the family had considered sending him to a mental institution. After Christmas in 1950, Claude was briefly brought back to the orphanage by his adoptive family for unspecified reasons, but was soon taken back in by them around half a year later in August 1951.

At the age of eight, Vivier was raped by his adoptive uncle, Joseph. He revealed this to a priest during a routine confession, and the priest reportedly told the young Claude that he would not be forgiven unless he told his parents. Vivier's parents became infuriated after he eventually recounted the sexual assault, believing he was either lying or responsible for the incident. This caused a significant strain in their relationship, and Vivier would ultimately spend less and less time interacting with his family. Despite this, Joseph's sexual abuse continued for years after. The family moved north to the suburb of Laval when Vivier was nine or ten, and frequently migrated from house to house as they continued to struggle financially. These near-constant moves depressed Vivier and he became ever more lonely: "I remember when I was a child and we moved house – I went around the streets looking for friends, but came back to the house with my head down, still with no friends".