In rugby union, a Grand Slam is a term used to describe when one team in the Six Nations Championship (or its Five Nations predecessor) beats all the others during one years competition). This means that the team has won all five of their Six Nations matches and is named the Grand Slam champion. The Grand Slam is the greatest honor a team can achieve in the Six Nations. The term is also used in the context of a Grand Slam tour, which refers to a touring side from South Africa, Australia, or New Zealand that plays fixtures against all four home nations (England, Ireland, Scotland, and Wales) during their tour. If the tourists win all of those matches, they are said to have achieved a Grand Slam).

To summarize, a Grand Slam in rugby is a term used to describe when a team wins all five of their Six Nations matches in a single years competition.