Accessory extensor pollicis longus tendons are rare tendon anomalies. This report describes the case of a 54-year-old man who presented with pain and difficulty extending his thumb. Upon initial examination, despite a history of trauma, he could extend the interphalangeal joint, but with limited range. During the surgical exploration, an accessory extensor pollicis longus tendon was unexpectedly discovered. This tendon ran parallel to the extensor pollicis longus on the radial side and then attached to the distal phalanx of the thumb. Accessory extensor tendons are more commonly found on the ulnar side than on the radial side of the hand; therefore, this case is notable due to the unusual location of the tendon. After appropriate repair surgery, the patient successfully regained a normal range of motion. While previous classifications of extensor tendon anomalies have focused on the relationship between the extensor indicis proprius and extensor digitorum communis, this case suggests that it is also necessary to consider the relationship with the extensor pollicis brevis tendon. This case highlights the importance of recognizing tendon variations, especially in trauma situations. Detecting such variations preoperatively using imaging can significantly impact the treatment plan and surgical approach.