His Majesty's Prison Belmarsh is a Category A men's prison located in Thamesmead, south-east London, England. Belmarsh Prison is run by His Majesty's Prison Service and is situated next to HMP Isis and HMP Thameside. The prison is used for high-profile prisoners, particularly those concerning national security. Within the grounds of the prison is the High Security Unit (HSU), consisting of 48 single cells.