Ibsen Andreas Nelsen (October 2, 1919 – July 19, 2001) was an American architect and urban planner active in the Pacific Northwest.

He was born to a Danish immigrant family in Ruskin, Nebraska, which fled west during the Dust Bowl and settled in Medford, Oregon.

Nelsen left to form his own practice in 1967, where he designed the Museum of Flight and a series of academic facilities for Western Washington University.

At twelve, he began learning carpentry and cabinetry while working for his father Julius Nelsen, the owner of a construction firm.

[1][2][3] He joined the Army following the United States' entry into World War II and was deployed to the southwest Pacific Theater, seeing combat as an infantryman in New Guinea.

He graduated with a Bachelor of Architecture degree in 1951, and moved to Seattle, Washington, where he was briefly employed as a draftsman by the firm Naramore, Bain, Brady & Johanson.

[2][5] He designed commissions such as alterations to the University of Washington's Playhouse Theatre and the lakeside residence of painter Morris Graves in Loleta, California; the latter was described by the San Francisco Chronicle as his most significant house.

He helped sculptor Isamu Noguchi receive commissions in the northwest, including for Sky Viewing Sculpture at Western Washington University.

[1][4][8] During the 1970s, Nelsen organized a social club dubbed the "Chowder Society", which met at his home to discuss Seattle urban development.

Members of the society included the future mayor Paul Schell and journalists Emmett Watson and David Brewster.