The sale of the 13.22 carat rock known simply as "The Blue" came on Wednesday, a day after Sotheby's sold a 100-carat flawless yellow diamond for $16.3 million -- with both sales including commission.
When the world's largest orange diamond went under the hammer last November at Christie's in Geneva, it raked in $35.5 million.
Such marquee sales underline the growing popularity of coloured diamonds. Once considered a curiosity, they are rarer than white diamonds and now attract higher prices per carat than even the most flawless, translucent stone.
The US jeweler Harry Winston bought the blue diamond, described by Christie's as the largest fancy vivid blue diamond in the world, from an anonymous seller.
"Fancy vivid" is the top rating for coloured diamonds.
The company plans to rename the 13.22-carat pear-shaped rock the "Winston Blue".
Harry Winston was bought by watch group Swatch last year.