Plant-thorn synovitis is an uncommon cause of arthritis and has not been reported in Asian countries. Zanthoxylum ailanthoides, an aromatic plant distributed in East Asia, is used as a spice in Taiwan. We reported a case of acute monoarthritis over the third metacarpo-phalangeal joint of the right hand after an injury by the thorn of the Zanthoxylum ailanthoides. A thorn foreign body of 2.7 mm in length was detected in the joint by high-resolution ultrasonography and led to a surgical synovectomy. The patient recovered completely after synovectomy. Plant-thorn synovitis is easily negligible. Early diagnosis is difficult and frequently delayed. The removal of thorn by synovectomy is the only curative treatment for plant-thorn synovitis. Compared to CT and MRI, ultrasonography is inexpensive, nonradioactive, repeatable, and easily accessible. It could detect foreign bodies that are smaller than 0.5 mm. High-resolution ultrasonography is a useful tool for detecting plant thorn and could promote early diagnosis.