Spitbank Fort in the Solent has been sold for around £0.5m to a consortium of three Hampshire businessmen who plan to market it as a venue for private hire.
The grade II listed, 22,000-ton, circular fort is located a mile from the entrance to Portsmouth Harbour.
It has previously been run as a location for events and tourist attraction between April and October, but the new owners intend to extend this limited season.
It currently attracts around 25,000 visitors with a seating capacity of 170 – which is also expected to be extended.