The fibularis tertius muscle (FTM) is a rare anatomic variation. The 5 prevalence of this exclusively human structure, which is found in the anterior 6 compartment of the leg, is often underestimated, and it is believed that foot and ankle 7 conditions are more difficult to manage in patients with an FTM. The aim of this study 8 was to assess the presence of the FTM palpation and determine whether its presence is 9 associated with an individual's sex, because the exact prevalence in males and females 10 is unclear. 11 Methods: This observational cross-sectional study included 481 people (23.49% men 12 and 76.51% women) with a mean age of 23.51 ± 5.369 years, who were recruited from 13 a Podiatric Medicine and Surgery Clinic in Spain. Data on routine demographic and 14 clinical factors were recorded, and the presence or absence of the FTM was determined 15 based on surface visual or palpated localization of the tendon (using a consistent 16 protocol). 17 Results: The FTM was present in 38.25% (184/481) of the participants. Furthermore, 18 FTM were present in 38.6% (142/481) of females and 37.2% (42/481) of males. 19 Conclusions: The study revealed that the presence of the FTM varies between 20 individuals and does not depend on an individual's sex. Significant differences in the 21 prevalence of the FTM between countries should be carefully evaluated rather than 22 generalizing the results of this Spanish study to other non-Spanish populations. Larger 23 numbers of participants should be enrolled in future studies in order to meet the 24 statistical criteria.