The 24-cover café bar opened its doors to the public for the Easter weekend in a new location on the boat, moored on the Thames near London Bridge, as part of a move to make the museum's food and drink offering more accessible to passing trade as well as museum-goers.
Will Sales, general manager for Sodexo Prestige at HMS Belfast, said: "We've always had a café on board but it was in the bowels of the ship and you had to have a ticket to get into it.
"The Imperial War Museum was redeveloping the ship and the ticketing area to include space for a café, so we used the opportunity to redesign the offering to make it appeal to more people.
"We've extended the core opening hours so it's open when people pass on their way to work and can pick up a coffee and breakfast if they want."
The licensed café, which is open from 8am to 10pm daily, serves grand cru coffee, cakes and 'board specials' such as a cheese board of British cheeses from Paxton & Whitfield and a fish board of Formans London cured salmon, gin gravadlax and Brixham crab sandwich, alongside a selection of wine and beer.
Upper Deck Bar
It will be joined this summer by the Upper Deck bar, a roof bar that will overlook the ship itself and have views of the surrounding area, which includes popular London landmarks Tower Bridge, the Tower of London and City Hall.
Sales said he hoped the bar, which will open seven days a week, will serve as a destination for a wide range of people as well as visitors to the attraction.
"We know there are lots of people who work in the area and lots of tourists and Londoners who are looking for somewhere different to go. This will give them another option," he said.
Sales has worked with a number of London-based food suppliers, such as Brindisa to source produce for the bar's tapas-style menu and with local breweries such as Meantime for the drinks.
HMS Belfast, which supported the D-Day landings during the Second World War, is now permanently moored between London Bridge and Tower Bridge and is the only surviving example of big gun armoured war ships built for the Royal Navy in the first half of the twentieth century.
Sodexo Prestige has held the contract to cater at HMS Belfast and at the Churchill War Rooms for more than 10 years.