Mating systems and patterns in reproductive success of fishes play an important role in ecology and evolution. While information on the reproductive ecology of many anadromous salmonids (Oncorhynchus spp.) is well-detailed, there is less information for non-anadromous species including the Yellowstone Cutthroat Trout (O. clarkii bouvieri), a species of recreational angling importance and conservation concern. Here, we used data from a parentage-based tagging study to describe the mating system of Yellowstone Cutthroat Trout from a spawning tributary of the South Fork Snake River, Idaho, and identify predictors of relative reproductive success. We detected evidence of monogamy, polygyny, and polyandry and showed that reproductive success was best explained by arrival time at the spawning ground and total length. Specifically, the largest adults arrived earliest in the season and produced a disproportionate number of offspring. Lastly, we estimated the effective number of breeders (N_b) and effective population size (N_e) and showed that while Nb was lower than Ne, both are sufficiently high to suggest Yellowstone Cutthroat Trout in Burns Creek represent a genetically stable and diverse population.