Sold for £30,000 (€35,000, $47,000) the dress was part of a capsule collection of nine Saint Laurent pieces, dating from 1962 to 1970, at a Christie's clothing sale stretching from the 18th century to the 1980s.
Patricia Frost, director of Christie's textile department, described the dress, inspired by the work of Dutch abstract artist Piet Mondrian, as "a magic carpet piece, it takes you right back to 1966."
The London sale came just ahead of the 50-year anniversary of the founding of Saint Laurent's fashion house, on December 4, 1961. The couture house closed in 2002 when the designer stood down.
Since his death in 2008, the mystique of Saint Laurent's name has sent prices flying at a string of record-smashing auctions of art and belongings accumulated by the designer and his partner Pierre Berge over the decades.
Christie's intended the sale in part as a retrospective of Saint Laurent's early career, at Dior from 1957 then striking out on his own aged 25.
"Saint Laurent lives on everywhere you look," said Berge, now 81, who co-founded his fashion house and helped run it for 40 years. "The most important are perhaps those you notice less," he told AFP.
"Saint Laurent completely defined his era. Everyone has a Saint Laurent – although they often don't realise it," said Berge, who today runs a foundation created in Saint Laurent's memory.
"He invented an entire masculine world that he fitted to women's bodies: tuxedos, sahara jackets, the sailor's jacket."
Berge's foundation has also sent travelling exhibitions of his work around the world, from Paris last year, to Madrid at the moment, and Denver, Colorado a few weeks from now.
From theatre to dance and cinema, Saint Laurent cultivated close ties with the arts, creating tribute collections from Matisse to Cocteau, Van Gogh or Picasso, as well as his Mondrian and pop art dresses.
Other pieces in the Christie's sale included an Indian-inspired green brocade coat, from Saint Laurent's first own-name catwalk show in January 1962, that was snapped on the cover of Elle magazine that year, and sold for £2,000.
"Saint Laurent's clothes happen to make people look wonderful but they also have more depth to them," Frost said. "They should be looked at more as works of art. They are not really for wearing, they are more for museums."