The exchange will not happen overnight: the complex logistics involved mean it will be 2017 before the great cetacean is hanging from the ceiling of the iconic Victorian Hintze Hall.
The museum thinks the change will increase the wow factor for visitors.
But it also believes the whale can better convey all the cutting-edge science conducted at the institution.
That is something a plaster-cast model of a Diplodocus skeleton - as familiar and as popular as it has become - can no longer do effectively.
"Everyone loves 'Dippy', but it's just a copy," commented Sir Michael Dixon, the NHM's director, "and what makes this museum special is that we have real objects from the natural world - over 80 million of them - and they enable our scientists and thousands like them from around the world to do real research."
