Ellen Lowenstein (nee Goodman, b. 1939/1940) is a St. Louis-based interior decorator, event planner, socialite, and activist. Lowenstein is the daughter of artist and arts patron Alice Hahn Goodman and Stanley Goodman, who served as CEO of the May Department Stores Co. and president of the board of the St. Louis Symphony Orchestra.

While straight herself, Lowenstein had a large circle of gay friends whom she met through her involvement in the local arts scene. An adept hostess and party planner throughout her life, in the late 1980s she was one of the founders of the Late Knights of Pythias. This was a primarily (although not exclusively) gay group that put on legendary parties for Halloween, Mardi Gras, and other occasions. These parties were usually held in otherwise vacant historic buildings, typically started around midnight and lasted until dawn, and featured elaborate, fantastical decorations and guests in creative and often risque costumes. The Late Knights of Pythias remained active until the mid-1990s.

During this same period, the AIDS crisis began to have a serious impact on St. Louis's gay community, and Lowenstein witnessed many of her friends fall ill and die. In response, she threw herself into the local community response to AIDS. She became involved in the NAMES Project and the AIDS Foundation of St. Louis and helped organize several successful fundraising events.

Daniel Clark Johnson was another founder of the Late Knights of Pythias, and he was responsible for the interior design of many of the events put on by the organization. He was also responsible for the design of the NAMES Project Quilt installation at Forest Park Community College in 1988. Johnson's partner, Patrick Leonard, was on the committee that worked to bring the quilt to St. Louis on its first national tour.