Ragni Piene (born 18 January 1947, Oslo) is a Norwegian mathematician.
Her research is within the field of algebraic geometry. She took her doctorate at the Massachusetts Institute of Technology in 1976 and was appointed professor at the University of Oslo in 1987. She is a member of the Norwegian Academy of Science and Letters. Since 2003 she has been a member of the executive committee of the International Mathematical Union. In 2012 she became a fellow of the American Mathematical Society.