The 'simple yet stylish' Tarlair tidal pool complex, which was designed by architect John C Miller and opened in 1931, was recently awarded category A listing by Historic Scotland.
The pool, east of Macduff in Aberdeenshire, is one of only three surviving outdoor seaside swimming pools north of the border and, according to Historic Scotland, the most important.
Despite being out of use since 1995, it is still regarded as the best 'intact' example of a 1930s seaside leisure complex in the country, and comprises a large swimming pool, paddling pool, boating pool and tea pavilion.
Debbie Mays, Historic Scotland's inspector of historic buildings, said: 'Outdoor pools were highly fashionable meeting places in the 1930s, especially at the seaside.
'They were built in quite large numbers, and have an important place in our social history. But few now survive, so it's great that we still have such a good example as the one at Tarlair.'
Miller designed the swimming pool's outer wall to be fractionally below high-tide level which meant waves rolled in over the edge, giving swimmers the pleasures of the sea in a controlled environment.
Mays added: 'The whole idea was to offer families lots of different ways to have fun at an elegant pool complex with a superb natural setting in a spectacular rocky bay.'
The other known surviving outdoor seaside swimming pools are at Stonehaven, listed Grade B, and at Gourock, which has been greatly altered and is no longer tidal.