Norwegian physical chemist and corecipient, with Derek H.R. Barton (Barton, Sir Derek H.R.) of Great Britain, of the 1969 Nobel Prize for Chemistry for his work in establishing conformational analysis (the study of the three-dimensional geometric structure of molecules).

Hassel studied at the University of Oslo and received his doctorate at the University of Berlin in 1924. He joined the faculty of the University of Oslo in 1925 and from 1934 to 1964 was a professor of physical chemistry and director of the physical chemistry department. He began intensive research on the structure of cyclohexane (a 6-carbon hydrocarbon molecule) and its derivatives in 1930 and discovered the existence of two forms of cyclohexane. At this time he set forth the basic tenets of conformational analysis and wrote Kristallchemie (1934; Crystal Chemistry). After the mid-1950s Hassel's research dealt mainly with the structure of organic halogen compounds.