Riccardo Giacconi (born October 6, 1931) is an Italian-born, naturalized American Nobel Prize-winning astrophysicist who laid the foundations of X-ray astronomy. He is currently a professor at the Johns Hopkins University.

Born in Genoa, Italy he received a degree from the University of Milan before moving to the US to pursue a career in astrophysics research. He became an American citizen.^{[not in citation given]}

Cosmic X-ray radiation is absorbed by the Earth's atmosphere, and so space-based telescopes are needed for X-ray astronomy. Giacconi worked on the instrumentation for X-ray astronomy, from rocket-borne detectors in the late 1950s and early 1960s, through to Uhuru, the first orbiting X-ray astronomy satellite, the Einstein Observatory, the first fully imaging X-ray telescope put into space, and the Chandra X-ray Observatory which was launched in 1999 and is still in operation. Giacconi also transferred his expertise to other fields of astronomy, including being the first director of the Space Telescope Science Institute, the science operations center for the Hubble Space Telescope.