Onslow Square is a garden square in South Kensington, London, England.
The square lies between the Old Brompton Road to the northwest and the Fulham Road to the southeast. To the north is South Kensington tube station. To the south is the Royal Marsden Hospital. As well as the main square, the address covers the street to the southwest that turns into Onslow Gardens, and the street to the northwest that meets Pelham Street by South Kensington tube station.
The houses were built by Charles James Freake, on land belonging to the Smith's Charity Estate. His building agreement with the trustees of the charity stipulated that they should be stuccoed, and constructed to elevational designs and specifications provided by the trustees' architect and surveyor, George Basevi. The first four houses in the square, numbers 1,3,5, and 7, were begun in September 1845, and were occupied by 1847. The whole square was completed by 1865.
After the building of the first four houses, and Basevi’s sudden death in 1845, the style of the houses diverged from the agreed designs, using exposed grey stock brick, with only the dressings being of stucco.
St Paul's Church in Onslow Square was created as a part of Sir Charles James Freake's development of the square. The church was built during 1859–60, designed by James Edmeston together with Charles Freake's office. In 1876, a church hall was constructed south of the church. This was extended in 1893. The church is now administered by Holy Trinity Brompton Church.
Sir Albert Hastings Markham, KCB (1841–1918), explorer, author, and officer in the Royal Navy, lived at 4 Onslow Square as a child.