Janos Hajdu (born 17 September 1948) is a Swedish/Hungarian scientist, who has made contributions to biochemistry, biophysics, and the science of X-ray free-electron lasers.

[13] He is a professor of molecular biophysics at Uppsala University and a leading scientist at the European Extreme Light Infrastructure ERIC in Prague.

In his early work, Hajdu exploited chemistry to determine the symmetry of multi-subunit protein complexes, and characterised structural transitions in these systems.

The turning point occurred in 1996, when Hajdu took up a chair at Uppsala University and set up a European research network to explore the physical limits of imaging.

Hajdu presented their findings to the US Department of Energy in 2000 as part of the scientific justification for building the first hard X-ray free-electron laser, the Linac Coherent Light Source (LCLS), at Stanford.