A runner-up is a participant who finishes in second place in any of a variety of competitive endeavors, especially sporting events and beauty pageants; in the latter instance, the term is applied to more than one of the highest-ranked non-winning contestants, the second-place finisher being designated "first runner-up", the third-place finisher "second runner-up", and so on.

While loosely acceptable for describing any second-place finisher in a sporting event, the "runner-up" label is more properly appended to one that finishes in that position as the result of having lost in the final round of an elimination tournament; specifically, its most frequent use is encountered in tennis, and refers to the player (or doubles team) that loses the final match; in most tennis tournaments, a testimonial award, often in the form of a plate, is given to the runner(s)-up following the final match, with the winner(s) receiving a trophy instead.

In American team sports, the term is usually avoided in official circles, because the team losing in the final round of the postseason playoffs will have had to have won the championship of a lesser entity as a condition for reaching the finals; in basketball, American football and Ice hockey this would have been a conference championship, and in baseball a league championship, or colloquially, the league "pennant". Consequently, the losing finalist will typically be referred to as the champion of its conference (or league in the case of baseball) rather than as a runner-up. Although the team that won the finals will also be a conference (or league in baseball) champion by necessity, it is usually not referred to as such because the title of "finals champion" carries more prestige.