BACKGROUND COVID-19 is treated using antiviral and immunosuppressive drugs. Therefore, patients treated for COVID-19 may have an increased risk of secondary infection and a masked inflammatory response. We present a case of a deep neck abscess caused by pyogenic sternoclavicular arthritis during treatment for COVID-19. CASE REPORT A 55-year-old man with COVID-19 was admitted to the hospital with hypoxemia. He was then treated with remdesivir, tocilizumab, and dexamethasone and was placed in the prone position. When his condition stabilized, pain in the left shoulder appeared. There was no fever or elevation in inflammation markers, and he was administered analgesics. However, the pain worsened and redness of the left neck appeared. Plain computed tomography (CT) showed swelling of the left neck muscles. Because cellulitis was suspected, he was treated with antibiotics, but his symptoms did not improve. Three days after the plain CT, contrast-enhanced CT showed sternoclavicular arthritis, deep neck abscess, and mediastinal abscess. Therefore, an emergency incisional drainage was performed under general anesthesia. Wound cleaning and drainage were continued after surgery, and after drainage tubes were removed, the patient was discharged on postoperative day 17. CONCLUSIONS Cervical infections after COVID-19 treatment have been reported in a few cases. Particularly, deep neck abscesses require more attention since they could be fatal if not treated immediately. If a secondary infection is suspected in a patient treated with immunosuppressive drugs for COVID-19, a thorough physical examination should be performed to avoid misdiagnosis.