The Defender in the America's Cup sailing competition is the team that holds the trophy and has been challenged by another union for the title return.

The America's Cup is a sailing competition and the oldest international competition still operating in any sport. America's Cup match races are held between two sailing yachts: one from the yacht club that currently holds the trophy and the other from the yacht club that is challenging for the cup. The winner is awarded the America's Cup trophy, informally known as the Auld Mug. Matches are held several years apart on dates agreed between the defender and the challenger. There is no fixed schedule, but the races have generally been held every three to four years. The most recent America's Cup match took place in March 2021.