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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
  • Ultima Ratio Regum, a historical roguelike game