Klaus Rajewsky (born 12 November 1936 in Frankfurt am Main) is a German immunologist, renowned for his work on B cells.

He studied medicine in Frankfurt, Munich and at the Pasteur Institute, Paris.

He researched Hodgkin's disease and the role of B cells within the immune system.

From 1995 to 2001 he was head of the Monterotondo station of the European Molecular Biology Laboratory near Rome.

In 2001 he started working at the Center for Blood Research at Harvard Medical School, Boston, where an additional focus of his work concerns RNAi, especially microRNAs, in conjunction with immune development and control.