Portman Square is a garden square in Marylebone, central London, surrounded by townhouses. It was specifically for private housing let on long leases having a ground rent by the Portman Estate, which owns the private communal gardens. It marks the western end of Wigmore Street, which connects it to Cavendish Square to the east.

A garden square is a type of communal garden in an urban area wholly or substantially surrounded by buildings; commonly, it continues to be applied to public and private parks formed after such a garden becomes accessible to the public at large. The archetypal garden square is surrounded by tall terraced houses and other types of townhouse. Because it is designed for the amenity of surrounding residents, it is subtly distinguished from a town square designed to be a public gathering place: due to its inherent private history, it may have a pattern of dedicated footpaths and tends to have considerably more plants than hard surfaces or large monuments.