Posterior tibial tendon (PTT) dysfunction is recognized as an etiology leading to acquired flatfoot in adults, causing significant functional loss. Many risk factors and systemic conditions have been proposed in literature. However, many patients present PTT dysfunction without any of these characteristics. This suggests that there could be a genetic influence associated with posterior tibial tendinopathy. The purpose of the present study is to investigate the association of the −1607 polymorphism in the promoter gene of MMP-1 and posterior tibial tendinopathy. The test group included 50 women, who presented PTT dysfunction grade 2 or 3, and who were submitted to surgical treatment, with histopathological examination of the tendon and magnetic resonance image (MRI) confirming tendinopathy, while the control group was 100 asymptomatic women who presented intact PTT at MRI. The results were analyzed using the chi-square test. The data showed a 75% incidence of the allele 1G and 62% of the genotype 1G/1G at the control group while, at the test group, they showed a 78% incidence of the allele 2G and 72% of the genotype 2G/2G (p < 0.001). The −1607 polymorphism of promoter gene of MMP-1 is associated with the posterior tibial tendinopathy in the studied population.