Management of partial-thickness rotator cuff tears is actually controversial. We treated a patient with a partial-thickness tear of the supraspinatus tendon by a series of four type I porcine collagen ultrasound-guided injections, at weekly intervals. At the same time the patient underwent physical therapy, consisting of motor re-education and proprioceptive exercises. The patient was assessed before the treatment and up to 18 months after the last injection by the Constant–Murley score, the Disability of Arm, Shoulder and Hand questionnaire and ultrasonography. Shoulder pain and functional limitation progressively improved and they almost completely disappeared at the last follow-up. Ultrasonography showed a gradual healing of the partial-thickness tear and a regeneration of the tendon structure. This is the first study on ultrasound-guided injections of type I porcine collagen for the treatment of partial-thickness rotator cuff tears. Future research should confirm the excellent result achieved in this case report.