XMM-Newton, also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second cornerstone mission of ESA's Horizon 2000 programme. Named after physicist and astronomer Sir Isaac Newton, the spacecraft is tasked with investigating interstellar X-ray sources, performing narrow- and broad-range spectroscopy, and performing the first simultaneous imaging of objects in both X-ray and optical (visible and ultraviolet) wavelengths.

Originally scheduled for a two-year mission, the spacecraft remains in good health and has received repeated mission extensions, most recently in November 2018, and is scheduled to operate until the end of 2020. It will probably receive a mission extension lasting until 2022. ESA plans to succeed XMM-Newton with the Advanced Telescope for High Energy Astrophysics (ATHE) XMM-Newton is similar to NASA's Chandra X-ray Observatory, also launched in 1999.