An upset is a victory by an underdog team. In the context of the NCAA Division I men's basketball tournament, a single-elimination tournament, this generally constitutes a lower seeded team defeating a higher-seeded (i.e., higher-ranked) team; a widely recognized upset is one performed by a team ranked substantially lower than its opponent.

This is the list of victories by teams seeded 10 or lower in the first round and second rounds of the tournament, as well as those by teams seeded 8 or 9 against 1 seeds in the second round, since it expanded to 64 teams in 1985; as these low-seeded teams were automatically paired against higher-seeded teams at the start of the tournament, their opening victories are almost always considered upsets (to date only three teams seeded 10 or lower in the first round won as betting favorites, and only one such team won in the first round as a pick 'em). Most victories by these teams in later rounds were usually against better seeded opponents as well. The list also includes victories by teams seeded 8 or lower in the Sweet 16 (the four regional semifinals), teams seeded 7 or lower in the Elite Eight (the four regional finals), and teams seeded 6 or lower in the Final Four. All teams are listed by athletic brand names they used at the time of their wins, which do not always match those in use today.

The NCAA defines a tournament "upset" as a victory by a team seeded 5 or more lines below its defeated opponent.

On March 16, 2018, the University of Maryland, Baltimore County (UMBC) Retrievers became the first 16-seed to upset a 1-seed when they defeated the Virginia Cavaliers 74–54 in the first round; this was followed by the Fairleigh Dickinson Knights defeating the Purdue Boilermakers 63–58 on March 17, 2023.