His Majesty's Prison Belmarsh is a Category A men's prison in Thamesmead, southeast London, England. The prison is used for high-profile cases, particularly those concerning national security. Within the grounds is the High Security Unit (HSU), which consists of 48 single cells. It is run by His Majesty's Prison Service. The prison has been called "Britain's Guantanamo Bay" due to the long-term detention of terrorism suspects without charge. Considered the toughest prison in the UK, Belmarsh is also notoriously known as "Hellmarsh" owing to the high number of physical and authority abuses reported by both the prison's inmates (including former politician Jeffrey Archer, who coined the name and was imprisoned there for four years for perjury), and by human rights activists.

Belmarsh Prison was built on part of the East site of the former Royal Arsenal in Woolwich, and became operational on 2 April 1991. It adjoins Woolwich Crown Court.

In 1991, the Double A Cat prisoner armed robber Ronnie Field, an associate of London gangland boss Joey Pyle, was the first person to be held in Belmarsh's High Security Unit, shortly followed by members of the Arif firm and those convicted of the £10 million Gatwick airport diamond heist.

Between 2001 and 2002, Belmarsh Prison was used to detain a number of people indefinitely without charge or trial under the provisions of Part 4 of the Anti-terrorism, Crime and Security Act 2001, leading it to be called the "British version of Guantanamo Bay". The Law Lords later ruled in A v Secretary of State for the Home Dept that such imprisonment was discriminatory and against the Human Rights Act 1998.

It is often used for the detention of prisoners for terrorist-related offences. In September 2006, 51 such prisoners were held.

In 2009, an archaeological dig on the site led to the discovery of a 6,000-year-old trackway, the oldest known wooden trackway in northern Europe with the exception of the Sweet Track near Glastonbury.

In November 2009, an inspection report from the Chief Inspector of Prisons criticised the "extremely high" amount of force used to control inmates. The report also stated that an unusually high number of prisoners had reported being intimidated or victimised by staff.

In 2010, HMP Isis Young Offenders Institution was opened within the perimeter wall.

Belmarsh is a Category A Prison holding inmates from all over England and Wales. It also houses other categories of prisoners from primarily the Central Criminal Court (Old Bailey) and Magistrates' Courts in South East London as well as Crown and Magistrates' Courts in South West Essex. Accommodation at the prison is a mixture of approximately 60% multi-occupancy cells and 40% single cells, distributed mainly across four residential units.

Inmates at Belmarsh are offered access to education, workshops and two gyms. The gym staff have a partnership with Charlton Athletic F.C. to deliver FA accredited coaching courses for prisoners.

A listener scheme for prisoners at risk from suicide or self-harm is in operation as is a support group providing advice on immigration law for foreign national prisoners.

Jenny Louis, governor of HM Prison Belmarsh, Britain’s first female black governor accused of bullying staff. The outcome of the investigation and any subsequent actions taken regarding these allegations have not been reported.

• Ali Harbi Ali, murdered MP David Amess
• Paul 'Des' Ballard, television presenter, caused death by dangerous driving, and various other crimes including rape
• David Copeland, neo-Nazi who bombed a gay pub, Brixton Market and Brick Lane
• Louis De Zoysa, shot dead an on-duty police sergeant at Croydon police station in September 2020.
• Delroy Grant, the "Night Stalker", a serial rapist and burglar, with perhaps over 100 offences
• Danyal Hussein, murdered two sisters in a London park
• Daniel Khalife, former UK Armed Forces soldier and prison escaper
• Stephen Port, the "Grindr Killer", serial killer and serial rapist
• Axel Rudakubana, murder suspect in the 2024 Southport stabbings
• Khairi Saadallah, stabbed three people to death in a Reading park, seriously injured others

• Hashem Abedi (moved to HM Prison Frankland)
• Michael Adebowale (moved to Broadmoor Hospital)
• Jonathan Aitken
• Abu Hamza al-Masri (sentenced to life imprisonment in the United States. Incarcerated at ADX Florence)
• Sudesh Amman
• Jeffrey Archer
• Julian Assange, accused by the United States of espionage.
• Ronnie Biggs
• Charles Bronson
• David Carrick (moved to HM Prison Full Sutton)
• Anjem Choudary
• Wayne Couzens (moved to HM Prison Frankland)
• Richard Huckle (later moved to HM Prison Full Sutton, where he was murdered)
• Ian Huntley (moved to HM Prison Frankland in 2008)
• Usman Khan
• Denis MacShane
• Thomas Mair (moved to HM Prison Frankland)
• Jordan McSweeney (moved to HM Prison Long Lartin)
• Darren Osborne, murdered one person and attempted murder of others by driving into them (moved to HM Prison Full Sutton)
• Maurice Robinson (moved to HM Prison Lowdham Grange)
• Tommy Robinson, anti-Islam campaigner and far-right activist
• Richard Tomlinson
• John Worboys (moved to HM Prison Wakefield)

• Ministry of Justice pages on HMP Belmarsh Archived 15 February 2012 at the Wayback Machine
• HMP Belmarsh – HM Inspectorate of Prisons Reports

51°29′47″N 0°05′34″E / 51.4964°N 0.0929°E / 51.4964; 0.0929