In association football, a player who has appeared for a football club during a knockout cup but subsequently transfers to another club is ineligible to play for the new club in the remainder of that season's cup competition. Such a player is said to be cup-tied, i.e. tied to their original club for the duration of the cup tournament. They become eligible for their new club in the following season.

The rule is intended to prevent teams which progress in the competition buying talented players from teams which have already been eliminated, in an attempt to increase their chances of winning. It also discourages players whose chief priority is winning a trophy from requesting a transfer once their team has been eliminated from the competition. Since the introduction of transfer windows, which the cup-tied rule pre-dates, some have criticised the rule as outdated. Nevertheless, it remains widely applied.

Almost all cup competitions worldwide operate a cup-tied rule, but leagues do not (as leagues do not eliminate teams during the season). Cup-tied players are only prevented from playing in that specific competition, so for example a player who is cup-tied in the FA Cup may still be eligible to play in the League Cup (or vice versa). UEFA competitions are an exception: because teams can switch between the UEFA Champions League and UEFA Europa League during the season, UEFA has a more complex system for determining whether a player is cup-tied in one or both of those competitions.