The "God Letter", written in German by Einstein in 1954 to philosopher Eric Gutkind, is regarded as a key manuscript in the debate over science and religion and is Einstein's clearest statement of his views on the universal search for the meaning of life.
The letter, which was sold at Christie's on Tuesday (Wednesday NZT), was written the year before the Nobel prize-winner's death. In it, Einstein said he was proud to be Jewish, but that his pride was not rooted in religion.
"The word God is for me nothing but the expression and product of human weaknesses", he writes.
Experts at Christie's auction house in NY had previously estimated the so-called "God letter" could sell for up to $1.5 million.
The letter had previously sold for $404,000 at Bloomsbury Auctions in London in 2008. "No interpretation, no matter how subtle, can (for me) change anything about this", the letter says.
The one-and-a-half-page letter had been estimated by Christie's to fetch between $1 million and $1.5 million.
"For me the unadulterated Jewish religion is, like all other religions, an incarnation of primitive superstition".
"As far as my experience goes, they are no better than other human groups, although they are protected from the worst cancers by a lack of power".
Several of Einstein's letters have sold at auction in recent memory, ranging in size and scope.
He said he believed in "Spinoza's God" - referring to Baruch Spinoza, a 17th-century Dutch thinker - "who reveals himself in the lawful harmony of the world, not in a God who concerns himself with the fate and the doings of mankind". "Otherwise I can not perceive anything "chosen" about them", the letter says.