Posterior tibial tendon (PTT) tears are the most common cause of unilateral flatfoot in adults. It is a chronic pathologic condition with a degenerative inflammatory character which usually affects middle-aged or elderly women, with no history of trauma. PTT tears occur more rarely in young patients, after an acute traumatic event. Clinically, PTT tears present with pain along the tendon, increased by weight bearing, swelling and functional limitation. PTT tears are surgically classified in three types: with tendon hypertrophy, with tendon atrophy and with complete tendon tear. The three different anatomicosurgical patterns require different therapeutic approaches. This work was aimed at investigating MR capabilities in recognizing the direct and indirect signs of PTT tears, assessing MR sensitivity in different types of tears. Fourteen patients with clinically suspected PTT tears and 10 control patients underwent MRI of the ankle (0.5 T, GE Vectra) following a study protocol including SE T1-weighted and FSE T2-weighted scans acquired on the axial and sagittal planes. In all patients, the transverse diameter, the shape and the section surface of the lesions were calculated on the axial planes at different levels; all these parameters were compared with those of the adjacent tendons, i.e., flexor longus ditae and flexor hallucis. Moreover, the probable presence was investigated of intratendineous signal intensity changes, of peritendineous synovial fluid and of indirect signs of PTT tear, i.e., the lack of longitudinal talonavicular alignment and the bulging of the medial navicular tubercle. According to MR results, the 14 patients were subdivided into 8 patients with type-I tears, 2 with type-II tears and 4 patients with type-III tears. The measures of healthy and injured tendons were in substantial agreement: in fewer cases, agreement was found also between symptomatic and control patients with intratendineous signal changes. Thus, the specificity of the two parameters was reduced. Finally, such indirect signs of PTT tear as a hypertrophic navicular tubercle and the lack of longitudinal talonavicular alignment exhibited good specificity, but sensitivity was barely sufficient. MR diagnoses were confirmed at surgery in 6 patients with type-II and type-III tears. In conclusion, MRI appears to be a suitable modality for recognizing and classifying PTT tears and a useful tool for the treatment and follow-up of these patients.