Multiple mating is common in Sea Turtles, and previous studies have suggested that levels of multiple paternity are higher in larger rookeries compared with smaller rookeries. As a result, smaller rookeries may be more vulnerable to the loss of genetic variation than would larger rookeries. The critically endangered Kemp's Ridley Sea Turtle (Lepidochelys kempii) has a single high-density rookery (∼5,000 females) in Rancho Nuevo, Mexico, where high levels of multiple paternity were reported (81% of nests). A secondary nesting colony was re-established on Padre Island National Seashore, Texas, USA, in the early 1980s and, by 2010, 140 nests were found along the entire Texas coast. The relatively small size of the Texas rookery led us to predict low levels of multiple paternity compared with Rancho Nuevo, and we tested our hypothesis by genotyping 158 hatchling tissue samples from 25 nests on South Padre Island (SPI) at 14 microsatellite loci. Full sibship reconstruction indicated that multiple paternity occurred in 47.6% of nests, a value that is significantly lower than the 81% of nests in Rancho Nuevo. Genetic diversity on SPI was high (H_O = 0.80) and similar to Rancho Nuevo, although there was a genetic signal of a population bottleneck at SPI. Multiple paternity, small geographic range, and extensive movements by individuals probably act to maintain high genetic variation in this species, despite severe population declines.