Rule 21 of the Gaelic Athletic Association (GAA) was a rule in force from 1897 to 2001 which banned members of the British security forces from membership of the GAA and thus from playing Gaelic games. The affected organisations included the British Armed Forces and the Royal Ulster Constabulary (RUC), and prior to partition, the Royal Irish Constabulary (RIC) and Dublin Metropolitan Police. As well as the RUC in Northern Ireland, it also applied to police forces in Great Britain, which affected London GAA and the other British GAA affiliates.
The rule was abolished after the establishment of the Police Service of Northern Ireland (PSNI) as part of the Northern Ireland peace process.