Background: The use of steroid injection for treatment of a full-thickness rotator cuff tear is still controversial. This study aimed to evaluate the effectiveness and safety of this treatment method. Methods: Twelve patients in Group 1 received an intra-substance injection into rupture area of supraspinatus tendon with Diprospan 1 cc (betamethasone disodium phosphate 2 mg and betamethasone dipropionate 5 mg) and 1% xylocaine 1 cc. Twelve patients in Group 2 received an injection with normal saline 1 cc and 1% xylocaine 1 cc. The rupture size was measured by sonography before the injection, 3 months after the injection, and 6 months after the injection. Shoulder Pain and Disability Index (SPADI) score and Pain Visual Analogue Scale (VAS) score were measured and compared between the two groups before the injection, 1 week after the injection, 3 months after the injection, and 6 months after the injection. Results: Pain and function improved more in Group 1 than in Group 2. The therapeutic effect lasted for at least 6 months in both groups. The size of the supraspinatus tendon rupture was not increased after injection in either group. Conclusions: Intra-substance injection into rupture area of supraspinatus tendon with steroid and xylocaine is effective to reduce pain and improve function in patients with full-thickness supraspinatus tendon rupture without increasing the size of the rupture. Trial registration: Current Controlled Trials ChiCTR1900026376, data of registration: 2019/10/05 retrospectively registered.