Jun-Ichi Igusa (井草 準一 Igusa Jun’ichi, 30 January 1924 − 25 November 2013) was a Japanese professor emeritus at Johns Hopkins University, known for his contributions to algebraic geometry and number theory. Igusa zeta-function, the Igusa quartic, Igusa subgroups, Igusa curves and Igusa varieties were named after him.
He was an invited speaker for the 1962 ICM in Stockholm.  He was awarded Japan's Order of the Sacred Treasure. In 2012 he became a fellow of the American Mathematical Society.
He has three sons, Kiyoshi, Takeru and Mitsuru. Takeru Igusa is a professor of civil engineering at Johns Hopkins University. Kiyoshi Igusa is a professor of mathematics at Brandeis University.
- Igusa, Jun-ichi (1972), Theta functions, Die Grundlehren der mathematischen Wissenschaften 194, Berlin, New York: Springer-Verlag, ISBN 978-3-540-05699-7, MR 0325625
- Igusa, Jun-ichi (1978), Forms of higher degree (PDF), Tata Institute of Fundamental Research Lectures on Mathematics and Physics 59, Bombay: Tata Institute of Fundamental Research, ISBN 978-0-387-08944-7, MR 546292
- Igusa, Jun-ichi (2000), An introduction to the theory of local zeta functions, AMS/IP Studies in Advanced Mathematics 14, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-2015-5, MR 1743467