Trunk endurance tests are generally performed in sagittal or frontal plane. However, trunk field tests that measure the endurance of the rotator muscles are lacking. In view of this situation, we developed a flexion-rotation trunk test (FRT test) to assess the oblique abdominal muscle endurance. This new field test consists mainly in performing the maximum number of upper trunk flexion and rotation movements (reps) possible in 90 seconds. The objectives of this study were to analyze the FRT test reliability and to examine the effect of both the repetition and sex on test results. Fifty-one recreationally trained men (n = 35) and women (n = 16) completed 4 trials of the FRT test (T1, T2, T3, and T4), separated by 7 days each. The scores increased significantly between T1 and T3 (p < 0.001), showing a clear learning effect, but the increase between T3 and T4 was only 4.25% (p = 0.108). The intraclass correlation coefficients (ICCs) between trials were ≥0.83 and the standard errors of measurement (SEMs) ≤7.54 reps. The ICCs between trials increased, and SEMs decreased with test repetition, reaching an ICC of 0.94 and an SEM of 6.46 reps between T3 and T4. The comparison between sexes showed a higher abdominal endurance in men when compared with that in women (p = 0.003), and also a higher learning effect in men, especially at the beginning of the study. These findings suggest that, the FRT test is a reliable field protocol that differentiates between the abdominal endurance of men and women. However, it is necessary to perform an extensive familiarization period before testing (at least 3 trials of practice) to make learning effect negligible.