Spontaneous patellar tendon rupture is a rare condition that usually occurs secondary to conditions, such as rheumatoid arthritis, systemic lupus erythematosus (SLE), and use of steroids and fluoroquinolones. This paper presents a full-thickness patellar tendon rupture detected with magnetic resonance imaging, which was performed due to pain and swelling that started spontaneously on the front side of the left knee without a history of any trauma, of a 35-year-old male patient who had been followed up for a diagnosis of SLE for approximately 4 months and who had started taking methylprednisolone 4 mg/day 4 months prior, used it for 1 month, and then stopped using it. In patients who are followed up for a diagnosis of SLE, it should be kept in mind that there is a risk of developing a spontaneous tendon rupture secondary to chronic inflammation and use of corticosteroids