The financial services company has tweeted pictures of clients and customers enjoying the venue. Attendees, who include cardholders, have been treated to coffee, drinks and barbeque food.
An American Express spokeswoman told C&IT: "With an increased number of American Express Cardmembers likely to be visiting London this summer, we wanted to find ways to enhance their experience of the city.
"We have a longstanding relationship with the National Theatre, and this summer the American Express Service Centre can be found in its lobby on the South Bank. It will offer a convenient central location for Cardmembers to access our expert concierge team, who can share specialist insider knowledge of London, assist with making reservations and help with account queries.
She added: "Cardmembers will also have exclusive access to The Deck, a stunning rooftop space with views across the Thames that holds barbecues on Wednesday evenings and Sunday afternoons, and where we’ll be offering Cardmembers complimentary morning coffees."
@AmexUK tweeted: "We've amazing views across the city and if the weather takes a turn for the worse our deck @NationalTheatre has a large covered seating area."
The Deck opened in 2008 and experienced a surge of corporate bookings last year from companies including ITV, Sainsbury's and Magners.