A prison cell or holding cell is a small room in a prison, or police station where a prisoner is held.

Prison cells are usually about 6 by 8 feet in size with steel or brick walls and one solid or barred door that locks from the outside. Many modern prison cells are pre-cast. Solid doors have a window that allows the prisoner to be observed from the outside. Furnishings and fixtures inside the cell are constructed so that they cannot be easily broken and are anchored to the walls or floor. Stainless steel lavatories and commodes are also used. This prevents vandalism or the making of weapons.

There are a number of prison and prison cell configurations, from simple police station holding cells to massive cell blocks in larger correctional facilities.

In the United Kingdom, cells in a police station are the responsibility of the Custody Sergeant, who also logs the detainees and allocates him or her an available cell. Custody Sergeants also ensure cells are clean and as germ-free as possible, in accordance with the Human Rights Act.

The term lock-up is often used for a holding cell in a police station or detention facility to temporarily hold detainees before they go before a magistrate or judge.

The practice of assigning only one inmate to each cell in a prison is called single-celling.