BACKGROUND In the treatment of pes planus, if the implant does not match the anatomical structures of the sinus tarsi, synovitis can develop, causing pain symptoms. In the interest of making clinical recommendations for extra-osseous talotarsal stabilization, the goal of the present study was to characterize the anatomical characteristics of the sinus tarsi in patients with pes planus using magnetic resonance imaging (MRI) scans. MATERIAL AND METHODS This was a retrospective study involving 56 pes planus patients and 56 healthy volunteers from January 2014 to May 2022. The sinus tarsi was measured for length and width, for angle with the coronal and sagittal axes, and for length of the subtalar implant. RESULTS All examined metrics showed a difference between pes planus patients and healthy participants, with the exception of the subtalar implant's length. The average sinus tarsi length and width among pes planus patients were 19.23 mm and 2.91 mm, respectively. The angle between the sinus tarsi and the coronal and sagittal axes was 21.418° and 25.077°, while the length of approach was 33.06 mm and 0.76°. The only gender differences that were statistically significant were in the length and width of the sinus tarsi. There were no notable variations between the left and right sides. CONCLUSIONS Sinus tarsi morphology may be impacted by pes planus. When treating patients with pes planus, clinicians should take these anatomical factors into consideration since they might more completely characterize the anatomical features of the sinus tarsi.