The book, one of five numbered volumes printed on hand-made Japanese Washi paper and published in 1913, fetched 535,500 euros at Sotheby's Paris auction house on Monday.
Its estimated price had been between 400,000 and 600,000 euros.
Three other copies of what bibliophiles describe as the "Proustian Holy Grail" are in the hands of a private collector, while the fourth disappeared, never to reappear, during World War II.
The volume sold on Monday had not been seen in public since 1942, when bookshop boss Roland Saucier bought it at the Drouot auction house.
Swann's Way is the first volume of Proust's masterpiece, Remembrance of Things Past.
Anyone who doubts its quality only has to consult an enthusiastic review written when it was published.
Swann's Way was a "little masterpiece", it said, adding that the book "blows away the soporific vapours" of other works on the market "like a fresh breeze".
"What Mr Proust sees, senses, is entirely original," the article enthused.
Except the unsigned the article was written by Proust himself.
He sent it to editors asking them to publish it anonymously and eventually paying 600 francs (2,000 euros in today's prices) for its publication.
This capacity for self-promotion is revealed in the documents attached to the book by its first owner, Louis Brun.
Brun was Proust's first publisher, agreeing to put out Swann's Way after several others had refused to do so, but only at the author's own expense.
Although he defected to Gallimard, who paid for the publication of the succeeding volumes following the surprise success of the first, Proust remained grateful to Brun and presented the precious volume to him with a hand-written dedication.
Brun had eight letters from Proust explaining his PR efforts attached to the volume.
They were typed so that Proust's handwriting did not give him away.
Proust was not alone in paying for favourable reviews, which was a widespread practice at the time, according to Sotheby's manuscript expert Benoît Puttemans.
"He spared no expense and did not shy away from what looks to us today like active corruption," Proust biographer Jean-YvesTadié writes in the Sotheby's catalogue.