Phlegmasia cerulea dolens (PCD) is a rare and severe form of deep venous thrombosis that is classically associated with the lower extremities. We report a case of upper extremity PCD developing abruptly in a 37-year-old female with an indwelling cardiac pacemaker who presented to the emergency department complaining of pain and paresthesias in her left arm, adjoining left chest wall, and inferior neck. Her condition was promptly diagnosed and successfully treated with intravenous unfractionated heparin and balloon venoplasty with catheter-directed thrombolysis without any known residual signs or symptoms at hospital discharge.