It was once used as a command centre, to help protect London from air raids during the Second World War. Now Brompton Road Tube station, which closed in 1934, has been sold for £53 million.
The abandoned building, which is close to Harrods in one of the most expensive corners of London, was put on the market in September.
The Ministry of Defence said that it was selling the land as part of cost-cutting efforts - a move that now appears to have paid off.
It is expected to be used for residential development, with the money from the sale going back into the defence budget.
The Piccadilly Line station was designed by English architect Leslie Green, and retains its original distinctive green and brown wall tiles.
It was opened in 1906, but closed in 1934, after London Underground decided it was no longer financially viable.
It was later taken over by the War Office during the Blitz and used to control anti-aircraft batteries to protect London from air raids.
The 28,000 sq ft property contains a drill hall, garages, offices and mess above ground and has recently been occupied by the London University Air Squadron (UAS), the London University Royal Naval Unit (URNU), and 46F Squadron Air Training Corps (Air cadets).
There are also several underground areas, previously used as part of the former Brompton Road Underground station, but they have been left largely unused since the end of the Second World War.
Today the only remnants of the site's military past are a large map of London, the MoD said. It is working with the National Archives to make sure it is recorded.
Announcing the £53 million sale, defence minister Andrew Murrison said: "The Ministry of Defence is committed to selling off its surplus land and property in order to provide the best possible value for money to the taxpayer.
"At the same time we take our role as a custodian of the nation's history very seriously and have been working to record the historic significance of the building."
Additional reporting by PA