The rug was sold by Christie's auction house Tuesday on behalf of the Newport Restoration Foundation. It had been expected to fetch up to $US1.5 million.
The rug, which measures 2.3m by 1.7m, had been purchased by the late tobacco heiress Doris Duke in 1990. She left it to the foundation when she died.
Elisabeth Parker, head of Christie's rugs and carpets department, says there are only two other known rugs like it. She calls it an "amazing work of art" and says it has an intricate floral design and an unusually large number of colours.
Christie's says the buyer prefers to remain anonymous.