Sister Thomas Welder, OSB (born Diane Marie Welder;[1] April 27, 1940 - June 22, 2020) was an American educator, academic administrator, and Benedictine nun. Born and raised in North Dakota, she entered Annunciation Monastery in 1959, at age 19. She began working at the Benedictine-sponsored Mary College in 1963 and served as its president from 1978 to 2009. Under Welder, the college expanded to become the University of Mary. She received North Dakota's highest honor, the Theodore Roosevelt Rough Rider Award, in 2004.