We present the discovery of the first known coorbital companion of Saturn. Observed by the Outer Solar System Origins Survey (OSSOS) for 4.5 years, the orbit of this object is determined to high precision, revealing that it currently is in “horseshoe” libration with the Saturn. This coorbital motion will last at least thousands of years but ends ~10 kyr from now; the object is thus another example of the already-known “transient coorbital” populations of the giant planets, with this being the first known example for Saturn. We present a theoretical steady state model of the scattering population of Trans-Neptunian origin in the giant planet region (2–34 au), including the temporary coorbital populations of the four giant planets. We expose this model to observational biases using survey simulations in order to compare the model to the real detections made by a set of well-characterized outer Solar System surveys. While the observed number of coorbitals relative to the scattering population is higher than predicted, we show that the number of observed transient coorbitals of each giant planet relative to each other is consistent with a transneptunian source.