Isa Irmgard Degener née Hansen was a German-American plant collector and botanist, specializing in agrostology. She is known as the coauthor of Flora Hawaiiensis.
Isa Hansen studied botany at the University of Berlin and worked there as an assistant to Hermann Otto Sleumer and Erich Werdermann. Later she worked at the Berlin Botanical Garden and specialized in the grasses. In 1952 she met Otto Degener, who had come to Berlin in search of a grass expert. They began a correspondence and married in January 1953. The couple worked together, collecting botanical specimens in the Hawaiian Islands. They also made an important expedition to Polynesia. Otto and Isa Degener were associated with the New York Botanical Garden for many years. The New York Botanical Garden appointed Otto Degener as Collaborator of Hawaiian Botany in 1935 and appointed Isa Degener as Honorary Collaborator of Hawaiian Botany in 1975.
The pair focused on the flora of Hawaii and with her help, often as co-author, Otto Degener would publish 10 books and over 400 journal articles on that topic.