DESY, short for Deutsches Elektronen-Synchrotron (English: German Electron Synchrotron), is a national research centre for fundamental science located in Hamburg and Zeuthen near Berlin in Germany. It operates particle accelerators used to investigate the structure, dynamics and function of matter, and conducts a broad spectrum of interdisciplinary scientific research in four main areas: particle and high energy physics; photon science; astroparticle physics; and the development, construction and operation of particle accelerators. Its name refers to its first project, an electron synchrotron. DESY is publicly financed by the Federal Republic of Germany and the Federal States of Hamburg and Brandenburg and is a member of the Helmholtz Association.

DESY's function is to conduct fundamental research for solely civil and peaceful purposes. It specialises in particle accelerator development, construction and operation, particle physics, astroparticle physics and photon science research to explore the fundamental relationships between the structure, dynamics and function of matter. In cooperation with its partner organisations, its photon science research spans surface physics, material science, chemistry, molecular biology, geophysics and medicine through the use of synchrotron radiation and free-electron lasers.

In addition to operating its own large accelerator facilities, DESY participates in many major international research projects, for example the European X-Ray Free-Electron Laser in Germany, the Large Hadron Collider in Switzerland, the Belle II experiment in Japan, the IceCube Neutrino Observatory at the South Pole and the worldwide Cherenkov Telescope Array Observatory.

DESY operates in two locations. The primary location is in the Bahrenfeld quarter of Hamburg. In 1992, DESY expanded to a second site in Zeuthen near Berlin.

The DESY Hamburg site is located in the quarter Bahrenfeld, in the west of the city in the district of Altona. Its main accelerators are located here.

Following German reunification, DESY expanded to a second site in Zeuthen near Berlin. In 1939, the German Postal Ministry founded a nuclear physics laboratory there. After World War II, the laboratory was first named "Institute X", to become the Institute for High Energy Physics (German: Institut für Hochenergiephysik IfH), the high-energy physics laboratory of the German Democratic Republic belonging to the Academy of Sciences of the GDR. The institute was merged with DESY on 1 January 1992. It focuses on parallel computing for theoretical particle physics, the development and construction of electron sources for X-ray lasers as well as astroparticle physics with a focus on gamma-ray and neutrino astronomy.

DESY employs about 3,000 staff members from more than 60 nations. Most staff work at the Hamburg site, with about 270 at the Zeuthen site. These numbers include more than 130 trainees in various industrial-technical professions and about 500 PhD students and postdocs supervised by DESY. In addition, there are numerous master students from various universities.

The research centre is a foundation under civil law financed by public funds. In 2024, DESY had an annual budget of about 359 million euros (according to the German federal budget plan as part of institutional funding). In addition, it had a project funding income of about 32 million euros. 90% of the annual budget is provided by the German Federal Ministry of Research, Technology and Space and 10% respectively by the Free and Hanseatic City of Hamburg and the German federal state of Brandenburg.