Una O'Connor (23 October, 1880 - 4 February, 1959) was a British actress who worked extensively in theatre before becoming a notable character actress in film.

Born Agnes Teresa McGlade to a Catholic nationalist family in Belfast, Ireland, and educated at St. Vincent's National School, she changed her name when she began her acting career with Dublin's Abbey Theatre.

For many years she worked in Ireland and England as a stage actress, and appeared in Alfred Hitchcock's Murder! (1930). Despite her lengthy apprenticeship she had not attracted much attention until she was chosen by Noel Coward to appear in Cavalcade (1933). Her success led her to Hollywood to reprise her role, and with its success, O'Connor decided to remain there.

A favourite of the director James Whale, among O'Connor's most successful and best remembered roles are her performances in Whale's The Invisible Man (1933) and Bride of Frankenstein (1935).

O'Connor's other notable films include The Barretts of Wimpole Street (1934), Stingaree (1935), The Personal History, Adventures, Experience, & Observation of David Copperfield the Younger (1935), The Informer (1935), Father Brown, Detective (1935), Little Lord Fauntleroy (1936), The Adventures of Robin Hood (1938), The Strawberry Blonde (1941), My Favourite Spy (1942), Random Harvest (1942), This Land Is Mine (1943), The Canterville Ghost (1944), Christmas in Connecticut (1945), The Bells of St. Mary's (1945), Cluny Brown (1946), Of Human Bondage (1946), and Adventures of Don Juan (1948).

O'Connor also appeared in supporting roles in various theater productions, and achieved an outstanding success in the role of "Janet McKenzie", the nearly deaf housemaid, in Agatha Christie's Witness for the Prosecution at Henry Miller's Theatre on Broadway from 1954 until 1956.

As one of the witnesses, in what was essentially a serious drama, O'Connor's character was intended to provide comic relief, as so many of her past characterisations had.

O'Connor was highly praised for her work, and also played the role in the Billy Wilder film version of the same name in 1957. The film was a great success, and O'Connor once again received excellent reviews. It was her final film performance. By this time she was in her late seventies, and decided to retire.

She died, never having married or had children, in New York City from heart disease, aged 78.

Úna O'Connor (born 1938) is a former Irish sportsperson who played senior camogie with Dublin from 1953 until 1975. She is regarded as one of the greatest players of all-time.