In response to sonar-guided attacking bats, some tiger moths make ultrasonic clicks of their own. The lepidopteran sounds have previously been shown to alert bats to some moths' toxic chemistry and also to startle bats unaccustomed to sonic prey. The moth sounds could also interfere with, or "jam," bat sonar, but evidence for such jamming has been inconclusive. Using ultrasonic recording and high-speed infrared videography of bat-moth interactions, we show that the palatable tiger moth Bertholdia trigona defends against attacking big brown bats (Eptesicus fuscus) using ultrasonic clicks that jam bat sonar. Sonar jamming extends the defensive repertoire available to prey in the long-standing evolutionary arms race between bats and insects.