The US embassy in Mayfair will be put up for sale almost immediately
The US embassy is planning to move from Mayfair's Grosvenor Square to the Nine Elms area in Wandsworth, south London.
Ambassador Robert Tuttle said security and environmental considerations, as well as the need for an embassy fit for the 21st Century, made the site ideal.
He described the UK as a "best friend" of the US and said the American authorities wanted to be close to the centre of government and parliament.
The deal is conditional on the approval of the US Congress and UK authorities.
The BBC's diplomatic correspondent James Robbins said Mayfair residents would be likely to welcome the move.
They had complained the security barriers around the present embassy, built after 9/11, would leave their homes susceptible to the greatest damage.
Mr Tuttle said he hoped the project would be self financing because the US had a valuable leasehold to sell in Grosvenor Square.
"This has been a long and careful process. We looked at all our options, including renovation of our current building on Grosvenor Square," he said.
"I'm excited about America playing a role in the regeneration of the South Bank of London," he added.
An international design competition will now be held, which the embassy said is intended to produce the best modern design, incorporating the latest energy efficient building techniques.
The current embassy is the largest such US institution in western Europe and one of London's most recognisable buildings.