Theta Upsilon (ΘΥ) was a national women's fraternity operating in the United States from February 1921 until May 1962, when the group was absorbed by the Delta Zeta sorority.

Theta Upsilon began as a local club at the University of California, Berkeley when six female students moved into a house on Walnut Street.

[1] On February 7, 1921, the Mekatina students renamed themselves inaugural members of the Alpha chapter of Theta Upsilon.

Ida Shaw Martin, of the Sorority Service Bureau, is credited with outlining the plans for Theta Upsilon and perfected its ritual.

Lambda Omega had begun as the Norroena club at Berkeley in 1915. its name meant "Breath of the North," with a ritual based on Native American and Norse values.

I pledge my heart, my head, and my hand to be a good citizen on the campus and in the community, to cooperate in all endeavors, to maintain high ideals in social life, to uphold a worthy standard in harmony with my sisters in Theta Upsilon, that together we may work to further the divine plan for all mankind."

[1] Theta Upsilon had installed a total of 38 chapters by the time of the merger, counting its own and those it absorbed from Lambda Omega.