Saint Laurent retired from haute couture in 2002 and French radio said he died at his Paris flat.
"He was someone who was very shy and introverted, who had only very few friends and hid himself from the world," Laurent Saint's long-time friend and business partner Pierre Berge told France Info radio. "(Gabrielle) Chanel gave women freedom. Yves Saint Laurent gave them power," said Berge, who gave no details of the death.
French President Nicolas Sarkozy praised Saint Laurent as a creative genius. "He was convinced that beauty was a luxury that every man and woman needed," Sarkozy said in a statement.
Saint Laurent was credited with changing forever what women wore and was the first designer to make luxury labels accessible to a wider audience through innovative read-to-wear collections.
He created signature looks, making the trouser-suit a daytime staple and the tuxedo an elegant option. He popularized safari jackets and thigh-high boots, and his transparent blouses made near-nudity acceptable in high society.
His square-shouldered suits became classics, and he simplified evening-wear, moving from shocking satins to flowing white crepe of Hellenic purity.
"He completely revamped a woman's wardrobe," luxury underwear designer Chantal Thomass told French radio. "What will stay is the tuxedo for women and the pants ... His fashion was full of color and inspired by art. He traveled a lot in his mind."
Fame And Trauma
The eldest child of a wealthy French industrialist, Saint Laurent was born and grew up in the then French colony of Algeria and showed a talent for design, making clothes for his younger sisters' dolls.
At 17 he entered a Paris fashion school, and his sketch for a cocktail dress won first prize in an annual contest.
Introduced to Christian Dior, the gangly Saint Laurent was hired on the spot by the creator of the "New Look" and became his chief assistant. On Dior's death in 1957, Saint Laurent became chief designer and swiftly outshone his mentor.
Success was immediate. After his first collection which introduced the widely-copied "trapeze" silhouette with narrow shoulders and flared skirt, the shy 21-year-old was pushed out on to the Dior building balcony and crowds in the Avenue Montaigne below hailed him like royalty.
Saint Laurent directed Dior for three years, until drafted for military service during the Algerian war.
For an over-sensitive youngster whose homosexuality had already made his school years a torture, army life was an unbearable ordeal. He had a nervous breakdown and spent nearly three months in a military hospital.
During Saint Laurent's convalescence, Berge persuaded a businessman to provide backing for the young designer to establish his own fashion house. Saint Laurent presented his first collection under his own name in 1962.
The "YSL" empire grew steadily and Saint Laurent showed an instinctive ability to sense what the mood on the streets was and turn it into high fashion.
But by the late 1980s Saint Laurent's collections had become overshadowed by his health problems -- he suffered from severe depression and underwent treatment for alcohol abuse.
Insiders said Saint Laurent, who never read newspapers or listened to the radio, became increasingly cut off from reality and lost touch with all but a tiny group of friends.
"Fame has destroyed him," Berge once said. "All designers have a bit of the megalomaniac in them -- the only difference is that the fake designers, the bad ones, are happy megalomaniacs while the real ones are unhappy megalomaniacs. Saint Laurent is an unhappy megalomaniac."
While Saint Laurent struggled with his personal demons, his fashion, perfumes and accessories business thrived. A 1989 flotation, the first by a fashion house, was a runaway success.
But when the Gulf War erupted and the world economy went into a slump, the house's fortunes took a turn too. Berge and Saint Laurent sank deeper in debt.
In 1992, YSL was absorbed by cosmetics and drugs company Sanofi, with Saint Laurent retaining creative control. Then in 1999 it was bought by the Gucci group, itself controlled by French luxury giant PPR.
Saint Laurent, rarely seen in public after his retirement, was awarded one of France's top honors in 2007.