There is no consensus on the use of intra-articular corticosteroid injections in diabetic frozen shoulder (FS). Thus, we aimed to compare clinical outcomes after intra-articular corticosteroid injections in patients with diabetic FS and idiopathic FS. Data collected from 142 FS patients who received glenohumeral joint intra-articular corticosteroid injections were retrospectively reviewed. Thirty-two patients were diagnosed with diabetic FS and 110 patients with idiopathic FS. Data including visual analog scale (VAS) for pain, American Shoulder and Elbow Surgeons (ASES) score, subjective shoulder value (SSV), and passive range of motion (ROM) were compared before the injection and at 3, 6, and 12 weeks after the injection. There were significant improvements in all outcomes (p < 0.001 for all parameters) through 12 weeks in both groups. There were no significant differences in all outcomes, except for ASES scores, between both groups at 3 weeks. However, there were significant differences in VAS score, SSVs, ASES scores, and passive ROMs, except for angle of abduction, between the two groups at 6 weeks and 12 weeks after injection. A single intra-articular steroid injection can be used as a conservative treatment for diabetic FS, but less effective than for idiopathic FS.