Introduction and Purpose Traditionally, linear periodization has been widely applied to endurance training programs, whereas reverse periodization has been often used for shorter endurance events (Clemente-Suárez et al., 2015; Clemente-Suárez et al., 2018; Clemente-Suárez & Ramos-Campo, 2019) and fitness (Prestes et al., 2009). The aim of this study was to describe the training characteristics of a Spanish high-level female ultrarunner in her preparation to the 2023 World Mountain and Trail Running Championships using a reverse linear periodization model. Methods Day-by-day data training of 1 study subject for 29 weeks (from November 2022 to June 2023) was retrospectively analyzed. Training form (endurance, strength or complementary sessions), intensity (low-, moderate-, and high-intensity training), intensity distribution, exercise endurance mode (cycling, elliptical, running or trail running) and specific exercise mode goal session (running uphill, running flat, running downhill, hiking or mixed) were considered. Specific sessions, key competitions, tapering, hypoxia period and periodization were also analyzed. Results An average training of 8 hr and 53 min per week, covering 87.2 km and 3,755 m of altitude gain, was completed. The hardest week consisted of 14 hr and 58 min, 145 km, and 7,552 m of altitude gain. Distribution percentage of the training form of total training time was ~60% of running, ~17% cross training (mainly road cycling) and ~11% of strength training. Intensity distribution varied considering specificity and characteristics of the main races, progressing from more polarized to pyramidal pattern distribution. Moreover, running volume increased during the preparation period and more specific sessions and with mixed specific exercise modes was realized close to the main races, with the longest trail run of 4 hr 18 min of duration. Training in hypoxia was also completed, and tapering started 12 days before the world championship. In addition to the World Championship, National Championship races were held during the preparation period, where a 4th and a 3rd place in the Marathon (February 2023) and Ultratrail (April 2023) National Championships, respectively, was achieved. Discussion Reverse periodization has been successfully implemented in an elite female ultrarunner, finishing 11th in the 2023 World Mountain and Trail Running Championships. Reverse periodization was planned and completed, mainly considering volume and tendency of distribution, going from a polarized pattern to a pyramidal one. Moreover, overall planning and key sessions progressed in specificity, developing more rolling-hill trail runs, longer trail runs, longer longest trail run of the week, and more specific exercise modes (i.e. more hiking and mixed sessions vs flats runs) along the macrocycle. Conclusion Training specificity in trail running, considering race intensity, exercise mode and race characteristics (i.e., elevation gain and distance) progressed during the macrocycle. Reverse periodization can be successfully implemented in ultratrail running by focusing on specific training and race demands, especially close to the main races.