Diane MacDonald Barz (August 18, 1943 – May 14, 2014) was an American judge. She was the first woman to serve as a member of the Montana Supreme Court, a position she held from September 1989 to 1990. She also served as an assistant United States attorney from 1991 to 1994. Her career covered many firsts for women in Montana legal history, and she also ran the Montana Youth Court for over fifteen years, where she founded the Youth Court Conference Committee.