BACKGROUND: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapidly spread across the world causing the coronavirus disease 2019. Due to high infectious spread, 2 messenger RNA vaccines, Pfizer-BioNTech and Moderna, were the first SARS-CoV-2 vaccines authorized in the United States. CASE REPORT: We report a rare case of a 19-year-old woman who developed isolated sacroiliitis following the third (booster) dose of the Moderna vaccination. CONCLUSIONS: While most vaccine adverse effects are self-resolved, we describe a unique case of isolated sacroiliitis after the SARS-CoV-2 vaccination, which was then successfully treated with sacroiliac joint injection after failing medical management. KEY WORDS: COVID-19, sacroiliitis, pain management, sacroiliac joint injection, Moderna, vaccination, case report