Acromioclavicular (AC) osteoarthritis (OA) is a frequent pathology of the shoulder in elderly patients. Drug injection plays an important role in treatment of AC OA. Literature has demonstrated excellent short-term results regarding shoulder function and pain. However, mid- to long-term results are lacking. The aim of this study was to assess the efficacy of a single intra-articular AC injection in patients with AC OA and to identify predictive factors for success. Methods: A retrospective study was performed to analyze success rate, shoulder function, and pain perception after a single intra-articular injection in patients with AC OA. Success was defined as the absence of reinterventions such as additional injection or surgery. Outcome measures were 1-year success rate and clinical outcome scores of Numeric Rating Scale (NRS) for pain, Oxford Shoulder Score, and Subjective Shoulder Value. Results: Ninety-eight patients participated in this study. At a median final follow-up of 0.8 years (interquartile range, 0–6), 57 of these patients (58%) had undergone a reintervention. The 1-year success rate was 47% (95% confidence interval, 37%–57%), with NRS at rest as the sole factor significantly associated with success. Thirty patients not requiring reintervention reported significant improvement from baseline for all reported outcome measures at final follow-up. Conclusions: AC injections offer a 1-year success rate of 47%. The AC injection produces good mid- to long-term clinical outcomes regarding shoulder function, quality of life, and pain perception in one-third of patients. Further research is essential to analyze mid- to longterm outcomes of AC injections. Level of evidence: Level IV.