Thomas Sandby (1721 – 25 June 1798) was an English map-maker who later became an architect and teacher. Along with his younger brother Paul, he became one of the founding members of the Royal Academy in 1768, and was its first professor of architecture.

Born in Nottingham, the sons of a textile worker, both the Sandby brothers joined the topographical drawing room of the Board of Ordnance at the Tower of London in the early 1740s. Thomas Sandby served at the Battle of Dettingen and the Battle of Culloden as draughtsman to Prince William Augustus, Duke of Cumberland. In 1747, the Duke arranged for him to be appointed Deputy Ranger of Windsor Great Park, where he began altering the park and laying out the artificial lake, bridge and grounds at Virginia Water. In 1759, he designed substantial extensions to the Park's Cumberland Lodge; related works include nearby Forest Lodge.

As well as being elected the Academy's first professor of architecture, Sandby was also appointed 'Architect of the King's works', and became a noted teacher of architectural theory.

Perhaps Sandby's most notable architectural commission was the design of the Freemason's Hall at Great Queen Street in central London, linking two houses purchased by the United Grand Lodge of England in 1775. Sandby won a competition to design a Grand Hall, which was additionally used for many social events (the Hall was extended in the 1820s by Sir John Soane, but was demolished in 1930 after suffering irreparable structural damage in a fire in 1883).