HM Prison Cardiff (Welsh: Carchar Caerdydd EF) is a Category B men's prison, located in the Adamsdown area of Cardiff, Wales. The prison is operated by His Majesty's Prison Service.

By 1814, the existing Cardiff Gaol was deemed insufficient for coping with both the scale of demand and quality of building to cope with the quickly expanding industrial town, and so proposals were made to build a new county jail for Glamorgan. Construction commenced in 1827, and the new stone building located south of Crockherbtown opened at the end of 1832, capable of housing 80 prisoners, including 20 debtors.

The three Victorian wings of Cardiff Prison underwent a major refurbishment programme in 1996, and the prison's capacity was extended by the commissioning of three new wings (C, D and E), with the number of places for life-sentenced prisoners increased also.

In 1997 Cardiff Prison was criticised for chaining sick inmates to their hospital beds after a probe into the death of one of Cardiff's prisoners. Three years later one of Cardiff's Assistant Governors was found dead after an investigation into child pornography. The manager had been arrested at the prison days earlier by detectives investigating the alleged misuse of a personal computer.

Cardiff Prison was criticised in 2001 for its cell-share policy in the wake of an inmate's death. Prison officials were advised to carry out full checks on prisoners on their arrival, to prevent violent or even fatal incidents from occurring. A year later the prison was criticised again for its poor record in dealing with drug abuse and providing decent recreation facilities for inmates.

A total of 20 judicial executions took place at Cardiff prison. The condemned prisoners were hanged for the crime of murder. Their names, ages and dates of execution are:

The remains of executed prisoners were buried in unmarked graves within the prison walls, as was customary. In late 2003, after capital punishment had been abolished in the UK, the remains of Corbett, Roberts, Grossley, Evans, Wills and Singh were exhumed from the prison grounds and reburied elsewhere in order to make space for the construction of a new cell block. The precise location of the new cell block is 51°28′47.58″N 3°10′04.68″W / 51.4798833°N 3.1679667°W / 51.4798833; -3.1679667.

The remains of Mahmood Mattan (executed in 1952 but cleared of murder in 1998) had previously been exhumed from the same location for reburial in 1996. Mattan is now buried in the Muslim section of Western Cemetery, Cardiff. His tombstone bears the epitaph "Killed by Injustice". After Mattan's conviction was quashed, his widow Laura and three sons (David, Omar and Mervyn) received the sum of £725,000 in compensation from the British Government.

Cardiff Prison accepts male adult prisoners remanded into custody who are drawn predominantly from the surrounding court catchment area of South Wales. In addition Cardiff also houses sentenced Category B and C prisoners.

Cardiff's regime includes full-time education, employment in the prison workshops, and training courses. There is a resettlement unit that offers prisoners various offending behaviour programmes and work based courses, and a Detoxification Unit accommodating 50 prisoners.

In 1999 the actor Keith Allen played a Probation and Parole officer at the prison, in the BBC television series Jack of Hearts.

• John Straffen

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