Although ulnar collateral ligament tear of the thumb is not uncommon, few reports have described this injury with magnetic resonance imaging (MRI) findings. We present a case of a 46-year-old woman who suffered from painful disability of her right thumb soon after a motorcycle accident. MRI of 2- mm slice thickness showed minimally retracted fibers of the torn ulnar collateral ligament of the right thumb, but the adductor aponeurosis was intact. A control study of the normal left thumb well delineated the course of the intact ulnar collateral ligament. Surgical findings revealed a nondisplaced ulnar collateral ligament tear near the proximal margin of the adductor aponeurosis. The ulnar collateral ligament was then repaired. Follow-up MRI at nine months after surgery showed a repaired ligament without evidence of re-tear. The patient’s symptoms were almost subsided.