The nine-storey building occupies a prominent position in an area which is being transformed into the city’s “cuisine quarter” with new restaurants opening on two sides of the square.
It faces Jenners department store on on side and the new Standard Life Aberdeen offices and Ivy restaurant.
Mischa Davis, Hanover fund manager at Rockspring, said: “The Edinburgh office market offers a compelling investment case with an undersupply of available space generating rental growth.”
The property Hanover is fully let and generates £1.4m rental income annually.
It was acquired by US investment business Mont Regis in 2000.
Smolka Strachan advised Mont Regis on the sale to Rockspring, which was advised by Cuthbert White.