Flanker is a position in the sport of rugby union. Each team of 15 players includes two flankers, who play in the forwards, and are generally classified as either blindside or openside flankers, numbers 6 and 7 respectively. The name comes from their position in a scrum in which they 'flank' each set of forwards. They compete for the ball – most commonly in rucks and mauls. Flankers also assist in pushing in a scrum, but are expected to detach from the scrum as soon as the ball is out to get to the play before the opposition's forwards. Flankers also participate in line-outs, either being lifted to contest or win possession, or to lift other players. Flankers are usually the key participants in the tackling process. The flankers, especially the openside, are often the fastest forwards on the team but still relied upon for tackling.

In the game of rugby union, there are 15 players on each team, comprising eight forwards and seven backs. In addition, there may be up to eight replacement players "on the bench", numbered 16–23. Players are not restricted to a single position, although they generally specialise in just one or two that suit their skills and body types. Players that play multiple positions are called "utility players".