To mark the occasion, the South Bank venue is receiving a lunchtime visit today from the Queen and Duke of Edinburgh, who will have a backstage tour hosted by NT artistic director Nicholas Hytner.
The royal couple will watch rehearsals for the forthcoming production of Emil and the Detectives, meet staff in the prop-making workshop and view a performance of a song from the musical Guys and Dolls.
The visit will conclude with the Queen unveiling a plaque to commemorate the theatre's 50th anniversary.
In its 50-year existence the National, which started life at the Old Vic before moving to its current home in 1976, has staged 800 productions.
A special 50th anniversary gala on 2 November will feature performances by some of the National's biggest stars over the years.
For more on the NT 50th celebrations, visit 50.nationaltheatre.org.uk