The luxury retailer has taken the spot in The Royal Exchange that was occupied by D&D London for 15 years, before the restaurant group decided not to renew its lease this summer.
It is the fourth UK site for Fortnum & Mason, which opened its original store in Piccadilly in 1707.
An announcement on behalf of the group says: “This opening marks the ongoing growth of [Fortnum & Mason] along with its commitment to London as a global finance capital.”
Its 90-cover restaurant has launched in The Royal Exchange courtyard, offering both table and counter dining.
Its all-day menu includes caviar, shellfish and smoked salmon hand-carved at the counter, alongside classic Fortnum’s dishes such as Welsh rarebit and knickerbocker glory.
There’s also champagnes, wines and cocktails and teas, including a special blend with Assam, Ceylon and Yunnan gold buds made exclusively for the Royal Exchange site.
A separate 2,368 sq ft retail space stocks items from its flagship Piccadilly store.
The Royal Exchange is a former City trading floor opposite The Bank of England, which now houses over 30 luxury retail stores set around a central courtyard.