Pseudoseptic arthritis is primarily described in rheumatoid arthritis and other systemic inflammatory conditions. To our knowledge, only 1 case report of pseudoseptic arthritis associated with intra-articular injection of a pneumococcal polyvalent vaccine (PPV) has been published. Here, a second case is presented in which a patient presented with swelling, pain, and erythema of the affected shoulder. A 59-year-old woman presented to the emergency department with a 3-day history of severe pain and decreased mobility of her left shoulder after receiving a PPV vaccination. Her clinical and laboratory workup was suspicious for septic arthritis; however, magnetic resonance imaging of the affected shoulder with and without contrast showed only a partial thickness tear of the rotator cuff, fluid in the subacromial/subdeltoid bursa, and subcutaneous edema without evidence of an abscess. Based on the clinical and laboratory data, she underwent arthroscopic debridement. There was inflammatory tissue throughout the shoulder but no obvious purulent material. She did well postoperatively with a supervised range of motion rehabilitation protocol. Her cultures remained negative. At 12 weeks, she was discharged from follow-up. We suspect that the vaccination was inadvertently injected into the glenohumeral joint directly through the rotator cuff given the lack of a full-thickness tear and the patient's thin body habitus, which could explain her aseptic inflammatory arthritis.