Colonel Eustace James Anthony Balfour (8 June 1854 – 14 February 1911) was a London-based Scottish architect. The brother of one British Prime Minister and nephew of another, his career was built on family connections. His mother was the daughter of a Marquess, and his wife Frances, a noted suffragist, was the daughter of a Duke. Frances's sister in-law was Princess Louise, daughter of the reigning Queen Victoria. Balfour was a senior officer of the Volunteer Force in London. His outspokenness on military matters was a factor in his appointment as an aide-de-camp to King Edward VII.