Beef trade was a major sticking point between the United States and South Korea in ratifying the KORUS FTA. The outcome of the renegotiations that led to the March 2018 agreement in principle did not impact sections pertaining to beef in the agreement, though looking at how beef trade would have been affected should the talks have failed highlights the importance of the agreement to both countries, but particularly the United States. This paper estimates the demand for imported beef in South Korea by source and product by using the production version of the Rotterdam demand system and assesses what the potential impact of U.S. withdrawal from the KORUS FTA would have been on beef trade between the U.S. and South Korea. The results suggest U.S. withdrawal from the KORUS FTA would have resulted in a considerable increase in Australian beef exports to South Korea, largely at the expense of U.S. beef. This is because there is significant price competition between beef imports from the United States and Australia. Furthermore, Korean consumers substitute American beef for Australian beef when the relative price of U.S. beef rises.