URR

URR or Urr may refer to:
 * Urea reduction ratio, used to quantify dialysis treatment adequacy
 * Urr Water, a river in Scotland
 * Haugh of Urr, a village by the Urr Water, in the Urr parish
 * Motte of Urr, a "motte-and-bailey" castle near the Haugh of Urr, beside the Urr Water
 * Union Railroad is a name, or part of a name, that has been used by many companies, mainly in the US
 * Union Railroad (Pennsylvania) is possibly the only current user of that name
 * Underground Railroad, an informal escape network used by 19th-century black slaves in the US
 * Unión de Rugby de Rosario, body that rules the game of rugby union in Rosario, Argentina
 * Ultima Ratio Regum, a historical roguelike game