A hypertrophied posterior tibial tendon (PTT) has been considered to be an important morphologic parameter of PTT dysfunction (PTTD). Previous research has demonstrated that the PTT thickness (PTTT) is correlated with early signs of PTTD. However, the thickness is different from hypertrophy. Thus, we devised the PTT cross-sectional area (PTTCSA) as a new predictive parameter for diagnosing the PTTD.The PTT data were acquired from 14 patients with PTTD and from 20 normal individuals who underwent ankle magnetic resonance imaging. We measured the PTTT and PTTCSA at the PTT on the ankle magnetic resonance imaging.The mean PTTT was 2.43 ± 0.39 mm in the normal group and 3.40 ± 0.42 mm in the PTTD group. The average PTTCSA was 16.10 ± 4.27 mm in the normal group and 26.93 ± 4.38 mm in the PTTD group. The receiver operator characteristic analysis curve demonstrated that the highest predictive value of the PTTT was 3.07 mm, with 85.7% sensitivity, 85.0% specificity. The highest predictive value of the PTTCSA was 22.54 mm, with 92.9% sensitivity, 90.0% specificity.Our findings suggest that the PTTCSA was a more valid predictor of PTTD, even though the PTTT and PTTCSA were both significantly associated with PTTD.