Postural therapies have emerged as effective non-invasive approaches to managing and preventing LBP in athletes. These therapies focus on correcting muscular imbalances, enhancing body awareness, and promoting proper alignment. Therefore, the objective of this study was to evaluate the effectiveness of implementing the Mézières method and isostretching postures in Spanish high-level national rhythmic gymnasts with low back pain (LBP). Specifically, the study aims to assess the impact of these postural therapies on functional limitations associated with LBP. This study was a parallel group, randomized controlled trial implemented in 17 rhythmic gymnastics athletes with low back pain conducted at baseline, at 2, 4, and 6 weeks, and upon completion of the 12-week intervention period. The outcome measures included pain measurement, the Borg effort scale, and trunk movement as flexion, extension, inclination, and rotation assessed using a wearable device sensor. The trial included eight participants in the Mézières group and nine in the isostretching group, with no significant age difference between the groups ( = 0.589). Significant differences were found for time ( = 0.000) and group ( = 0.001), indicating variations in left trunk inclination performance over time and between the groups. Both groups showed increased trunk flexion, with the Mézières group demonstrating higher values at all time points. Right trunk rotation fluctuated, with a notable increase in the Mézières group at 6 weeks. The Mézières group also showed higher left trunk rotation values, peaking at 6 weeks. Although the Mézières method showed certain advantages in right trunk rotation and left tilt, the results overall suggest that both approaches are effective in the specific context of this population.