Frei Paul Otto (31 May 1925 - 9 March 2015) was a German architect and structural engineer. Shortly before his death, he was awarded the Pritzker Prize of Architecture. Otto was born in Siegmar (since 1950 a part of Chemnitz). He studied architecture in Berlin before being drafted into the Luftwaffe as a fighter pilot in the last years of World War II. It is said that he was interned in a French POW camp and, with his aviation engineering training and lack of material and an urgent need for housing, began experimenting with tents for shelter. After the war he studied briefly in the United States and visited Erich Mendelsohn, Mies van der Rohe, Richard Neutra, and Frank Lloyd Wright. He began private practice in Germany in 1952. His saddle-shaped cable-net music pavilion at the Bundesgartenschau (Federal Garden Exposition) in Kassel brought him his first significant attention. He earned a doctorate about tensioned constructions in 1954.

Otto is the world's leading authority on lightweight tensile and membrane structures, and has pioneered advances in structural mathematics and civil engineering. Otto's career bears a similarity to Richard Buckminster Fuller's architectural experiments: both taught at Washington University in St. Louis in the late 1950s, both were architects of major pavilions at the Montreal Expo of 1967, both were concerned with space frames and structural efficiency, and both experimented with inflatable buildings. The work of both men go far beyond traditional methods of calculating structural stresses. His designs are regarded to have been heavily influenced by Australian architect Barry Patten, and his most famous design, the Myer Music Bowl (1959) in Melbourne.

Otto founded the famous Institute for Lightweight Structures at the University of Stuttgart in 1964 and headed the institute till his retirement as university professor. Major works include the West German Pavilion at the Montreal Expo in 1967 and the roof of the 1972 Munich Olympic Arena, inspired by Vladimir Shukhovs architecture.

Noteworthy is the international Architecture Symposium "Mensch und Raum" (Man and Space) at the Vienna University of Technology (Technische Universitat Wien) in 1984 which received international attention. Otto participated,and with him among others: Justus Dahinden, Dennis Sharp, Bruno Zevi, Jorge Glusberg, Otto Kapfinger, Paolo Soleri, Pierre Vago, Ernst Gisel, Ionel Schein.

Otto's unusual first name Frei (in English "free") was given to him by his mother who perceived freedom as a fundamental value.