The National Library of Wales was designed by Sidney Kyffin Greenslade, assisted by Reginald Bloomfield, and construction was carried out in stages under Adams, Holden and Pearson. Occupation started in 1916; the central block opened in 1937 and the final section of the original design opened in 1955.

The complex has a square plan composed of four three-storey ranges around four rectangular courtyards formed by the central entrance hall, main staircase and transverse corridor link ranges. The exterior is in an Art Deco/Greek Classical style; the west and south fronts are faced in Portland stone and Cornish granite, the north and east are brick with Portland dressing. The roofs are half hipped with glazing to the top. The main block has 11-bays and the central 5-bays are advanced. There is a recessed central entrance bay with fluted Ionic columns flanking the second floor window frame and a balcony brought forward over the porch.
There are workshops (nprn 406697) to the north side.

RCAHMW, 06 November 2008.