The sex difference in marathon running is increased with lower participation of women than men, but whether this occurs for ultramarathon running is not known. The study purpose was to determine whether the sex difference in performance widens among lower-placed runners and the association between the sex difference in running speed and participation rates. The top-10 ultramarathon running times, age at performance date, and the number of men and women finishers were analyzed from 20 races (45–160 km) in the US Track and Field Ultra Running Grand Prix. Men were faster than women for all events (18.7% ± 5.8%, P < .001). The sex difference in speed was the least for 100 km (14.9% ± 4.2%) and greatest for 45–50 km (19.3% ± 5.8%). The top-10 men were younger than the top-10 women (37.7 ± 3.2 and 39.0 ± 3.1 y, respectively, P < .001). The sex difference in speed increased with finishing place (1st place 15.6% ± 6.6% vs 10th 20.8% ± 5.6%, P < .001). Association analysis showed that the sex difference in speed was largest when there were fewer women than men finishers in a race; the strength of the association was greatest for the 80-km distance and least for the 160-km. Lower participation rates of women than men in the lower-distance ultramarathons and less depth among lower-placed women runners inflate the sex difference in ultramarathon performance.