Bryanston Square is an early 19th-century garden square built on the Portman Estate. Montagu Square and Bryanston Square were the chief compositions of the estate between Gloucester Place and Edgware Road. The garden was provided for residents of surrounding houses. It retains notable London plane trees, mainly around the perimeter but also on the central lawn, and the path layout is recognisably descended from the original symmetrically undulating perimeter.​

Bryanston Square and Montagu Square to the east were begun by the builder David Porter in c.1811 for the Portman Estate, designed by Joseph T Parkinson, District Surveyor of Westminster. Bryanston Square was completed in c.1821 and is named after the Portman family seat near Blandford in Dorset. Parkinson’s facades were designed to mirror each other, but have subsequently been altered; the north end of the east side has been rebuilt and the centre of the west side remodelled. Although mocked by Charles Knight in 1844 as `twin deformities…long narrow strips of ground, fenced in by two monotonous rows of flat houses’, Bryanston and Montagu were not, in fact, a pair. Bryanston is wider and plays a significant part in the surrounding townscape, being aligned as part of an axis with Marble Arch to the south and Wyndham Place to the north. Smirke’s St Mary’s Church, begun in 1823, situated at the northern end of Wyndham Place, acts as an eye-catcher, closing a long vista from Oxford Street through the square.

The garden was laid out in 1810 and was provided for the residents of the surrounding houses. It continues to be administered by a Trust set up by an Act of Parliament of 1813, which has a statutory obligation to maintain the garden and its surrounding railings, levying a rate for the purpose on the occupants of Bryanston Square. In 1928 the garden was described as ‘a long narrow enclosure surrounded by a thick shrubbery’ ‘laid out as lawns with some fine trees’.

Today the garden retains notable London plane trees, mainly around the perimeter but also on the central lawn, as shown on the OS Map of 1870. The trees now obscure the axial view from Marble Arch to St. Mary’s Church. The path layout is recognisably descended from the symmetrically undulating perimeter path illustrated in Potter’s Marylebone Survey of c.1832. The circular beds edged with C19th tiles appear to be vestiges of the original elements. The perimeter is planted sporadically with shrubs and perennials, but modern gardening is agreeably low-key. The dwarf weeping ash in the middle of the lawn, which Knight scorned as resembling `a gigantic umbrella or toad-stool’ has since gone. There are C19th edging tiles in places around the paths, and benches made in c.1954 from the timbers of Admiral Lord Jellicoe’s flagship. The square is railed, and contains a modest C20th timber-framed shelter; the original iron railings were removed in WWII and replaced with wire fencing, but new railings were restored in 1980. There are 34 London plane trees in the garden, about a third of which are survivals from the last quarter of the C19th. Other trees include cherry, malus, lilac, laburnum, false acacia and Japanese maple.