Gareth Wyn Williams (26 September 1978 – c. 16 August 2010) was a Welsh mathematician and Junior Analyst for GCHQ on secondment to the Secret Intelligence Service (SIS or MI6) who was found dead in suspicious circumstances in Pimlico, London, on 23 August 2010, at a flat used to house Security Service's staff. The inquest found that his death was "unnatural and likely to have been criminally mediated." A subsequent Metropolitan Police re-investigation concluded that Williams's death was "probably an accident".

Two senior British police sources have said some of Williams's work concerned Russia – and one confirmed reports that he had been helping the US National Security Agency trace international money-laundering routes that are used by organised crime groups including Moscow-based mafia cells.

Originally from Valley, Anglesey, Wales, Williams, who spoke Welsh as a first language, began studying mathematics part-time at Bangor University, while still attending his secondary school, Ysgol Uwchradd Bodedern, and graduated with a first-class degree at age 17. After gaining a PhD at the University of Manchester, after failing an exam, he dropped out from a subsequent post-graduate course at St Catharine's College, Cambridge, and took up employment with GCHQ in Cheltenham in 2001, renting a room for nearly a decade in Prestbury, Gloucestershire. Reportedly an intensely private man and a keen cyclist, Williams was due to return to Cheltenham on 3 September 2010, after spending a year in London, following his annual leave.

Police visited Williams's home during the afternoon of Monday 23 August 2010, as a "welfare check" after colleagues noted he had been out of contact for several days. His naked, decomposing remains were found in the bath of the main bedroom's en-suite bathroom, inside a red sports bag that was padlocked from the outside, with the keys inside the bag. The local police had gained entry into the top floor flat at 36 Alderney Street, Pimlico at around 16:48; on examination of the bag, the premises were declared a crime scene. His family alleged that crucial DNA was interfered with and that fingerprints left at the scene were removed as part of a cover-up. Inconclusive fragments of DNA components from at least two other individuals were found on the bag. A forensic examination of Williams's flat has concluded that there was no sign of forced entry or of DNA that pointed to a third party present at the time of his death.

Scotland Yard's inquiry also found no evidence of Williams's fingerprints on the padlock of the bag or the rim of the bath, which the coroner said supported her assertion of "third-party involvement" in the death. Metropolitan Police Deputy Assistant Commissioner Martin Hewitt said it was theoretically possible for Williams to lower himself into the bag without touching the rim of the bath. A key to the padlock was inside the bag, underneath his body.

Williams was buried at Ynys Wen cemetery in Valley, Anglesey, on 26 September 2010, following a private funeral service at Bethel Chapel in Holyhead attended by his family, friends, former colleagues in the intelligence services, and also by the head of SIS, Sir John Sawers.

Williams's date of death was estimated to have been in the early hours of 16 August, one week before he was found.

Soon after the investigation started, the heads of the Secret Intelligence Service and Metropolitan Police met to discuss how the police would handle the investigation in light of the top secret nature of Williams's work, and who would lead the investigation. Williams had recently qualified for operational deployment, and had worked with U.S. National Security Agency and FBI agents. The U.S. State Department asked that no details of Williams's work should emerge at the inquest. The Foreign Secretary, William Hague, signed a public-interest immunity certificate authorising the withholding from the inquest of details of Williams's work and U.S. joint operations.