Achievement (1864–1872) was a British Thoroughbred racehorse and broodmare. In a career that lasted from April 1866 to June 1868 she ran twenty-three times and won sixteen races. She was arguably the best British two-year-old of either sex in 1866 when she won eleven times and was only beaten when attempting to concede weight to colts. As a three-year-old she won the Classic 1000 Guineas at Newmarket and then returned from a surprising defeat in The Oaks to beat The Derby winner Hermit in the St Leger at Doncaster. After failing to show her best form in 1868 she was retired to stud and died four years later.