Spontaneous tendon rupture is a rare entity. Herein, we report a case of spontaneous rupture of the extensor pollicis longus (EPL) tendon in a 61-year-old woman with systemic lupus erythematosus (SLE). The patient worked as an aesthetician and had a 42-year history of SLE that was well controlled using oral steroids. She presented with an inability to extend her left thumb, with no prior history of trauma or symptoms. On imaging, there was no evidence of degenerative changes or osteophyte formation in the gliding area of the EPL tendon. Intraoperatively, there was evidence of tendon rupture around Lister's tubercle. We performed a tendon transfer of the extensor indicis proprius for repair. The patient recovered active thumb extension and returned to work 4 months after surgery without any complications. We suspected that spontaneous EPL tendon rupture was caused by a combination of disease-related factors, including long-term steroid use, chronic inflammation, and continuous mechanical stress from her work and daily activity. This case report demonstrates the multifactorial aetiology of spontaneous tendon rupture in patients with SLE and the importance of monitoring for this complication during routine follow-up.