Trojan objects share a planet's orbit, never straying far from the triangular Lagrangian points, 60° ahead of (L4) or behind (L5) the planet. We report the detection of a Uranian Trojan; in our numerical integrations, 2011 QF99 oscillates around the Uranian L4 Lagrange point for >70,000 years and remains co-orbital for ~1 million years before becoming a Centaur. We constructed a Centaur model, supplied from the transneptunian region, to estimate temporary co-orbital capture frequency and duration (to a factor of 2 accuracy), finding that at any time 0.4 and 2.8% of the population will be Uranian and Neptunian co-orbitals, respectively. The co-orbital fraction (~2.4%) among Centaurs in the International Astronomical Union Minor Planet Centre database is thus as expected under transneptunian supply.