Alpha Delta Theta (ΑΔΘ) was a national collegiate sorority operating in the United States from 1919 to 1939. The sorority officially affiliated with Phi Mu fraternity on August 30, 1939.

Ten female students at Transylvania College in Lexington, Kentucky formed the local women's fraternity, Alpha Theta, in the fall of 1919. Its founders were students Willibel Chilton, Irene Duncan, Ruth Dutt, Valleria Grannis, Hazel Grow, Martha Hall, Zenaide Harold, Juanita Minish, Mary Owsley, and Violet Young, along with faculty member Isabel Wolf Hemenway.

At the time of Alpha Theta's formation, there were only two national women's fraternities in the United States. Alpha Theta approached one of the national sororities about membership; however, their request was denied because of Alpha Theta's affiliation with Hamilton, a junior college for women. Having heard that the National Panhellenic Conference "declared the number of national sororities was too small", the Alpha Thetas decided to form their own national group. On November 10, 1921, Alpha Theta became Alpha Delta Theta, though The Pledge Handbook of Alpha Delta Theta cites the founding date as November 10, 1919.

Alpha Delta Theta established its second chapter, Beta, in June 1922 at the University of Kentucky. This was followed by Gamma at the University of Cincinnati, Delta at the University of Illinois, Epsilon at Butler College, and Zeta at the University of Nebraska in 1923.

Within three years of existence, Alpha Delta Theta created eight collegiate chapters. The sorority was granted "associate membership" by the National Panhellenic Conference in October 1923. Three years later, in 1926, the sorority had full NPC membership. The new national sorority was admitted to the NPC in 1926. "It was admitted to the Congress at an earlier date in its history than any other fraternity". On March 15, 1926, Alpha Delta Theta was incorporated as a national organization in the state of Kentucky.

Alpha Delta Theta grew quickly and with much strength in its twenty years of existence. In 1930, it had 17 chapters and 1,258 members. At the time, four of its chapters owned houses and four others had purchased land for construction. In 1931, it had 18 chapters and 1,500 members. By 1932, Alpha Delta Theta had installed 25 collegiate chapters and 21 alumnae groups. The 1932–1933 academic year was especially noteworthy for Alpha Delta Theta, as "it had the distinction of breaking all fraternity records in scholarship... when 34 percent of its chapters held first place in scholarship on their respective campuses".

Alpha Delta Theta was governed by a grand council that was elected by chapter delegates to national biennial conventions. The grand council consisted of the grand president, grand vice-president, grand secretary, grand treasurer, and the editor of the Alpha Delta Theta magazine. The fraternity held its first two national conventions in Lexington, Kentucky. In 1931, its central office was in Little Neck, New York.

Despite these gains, Alpha Delta Theta had come into its flowering somewhat late when compared to the other established chapters on its campuses and thus faced several difficulties.