Sigurdur Helgason (Icelandic: Sigurður Helgason; 30 September 1927 – 3 December 2023) was an Icelandic mathematician whose research has been devoted to the geometry and analysis on symmetric spaces. In particular, he used new integral geometric methods to establish fundamental existence theorems for differential equations on symmetric spaces as well as some new results on the representations of their isometry groups. He also introduced a Fourier transform on these spaces and proved the principal theorems for this transform, the inversion formula, the Plancherel theorem and the analog of the Paley–Wiener theorem.

Sigurdur Helgason was born in Akureyri, Iceland on 30 September 1927. In 1954, he earned a PhD from Princeton University under Salomon Bochner. Helgason became a professor of mathematics at the Massachusetts Institute of Technology in 1965,^[1] and he retired from the faculty in 2014.^[2]

Helgason received the Børge Jessen Diploma Award of the Danish Mathematical Society in 1982, and the Grand Knight's Cross (Stórriddarakross) of the Icelandic Order of the Falcon in 1991.^[2][3] He was winner of the 1988 Leroy P. Steele Prize for Seminal Contributions for his books Groups and Geometric Analysis and Differential Geometry, Lie Groups and Symmetric Spaces.^[4] This was followed by the 2008 book Geometric Analysis on Symmetric Spaces. On 31 May 1996, Helgason received an honorary doctorate from the Faculty of Science and Technology at Uppsala University, Sweden.^[5]

Helgason was elected a member of the Icelandic Academy of Sciences [is] in 1960, the American Academy of Arts and Sciences in 1970,^[6] and the Royal Danish Academy of Sciences and Letters in 1972.^[7][3] In 2013, he became a fellow of the American Mathematical Society.^[8] He was made an honorary member of the Icelandic Mathematical Society when he turned 70, and a symposium was held in his honor.^[3]

Helgason died on 3 December 2023, at the age of 96.^[9]