The Messi–Ronaldo rivalry is a supposed association football rivalry between Argentine forward Lionel Messi and Portuguese forward Cristiano Ronaldo. Having won a combined eight FIFA Ballon d'Or awards, both Messi and Ronaldo are widely regarded as the best players in the world of their generation, as well as being among the best of all time in the sport. Both players have scored in two UEFA Champions League finals, have regularly broken the 50 goal barrier in a single season, and have scored over 500 goals each in their careers for club and country. Sports journalists and pundits regularly argue the individual merits of both players in an attempt to establish who they believe is the best player in modern football. It has been compared to legendary global sports rivalries such as the Muhammad Ali–Joe Frazier rivalry in boxing, the Björn Borg–John McEnroe rivalry in tennis, and the Ayrton Senna–Alain Prost rivalry in Formula One motor racing.

In a 2012 interview, Ronaldo commented on the rivalry, saying "I think we push each other sometimes in the competition, this is why the competition is so high", while Ronaldo's manager during his time at Manchester United, Alex Ferguson, opined that "I don't think the rivalry against each other bothers them. I think they have their own personal pride in terms of wanting to be the best". Messi himself denied any rivalry, saying that it was "only the media, the press, who wants us to be at loggerheads but I've never fought with Cristiano". In response to the claims that he and Messi do not get on well on a personal level, Ronaldo commented; "We don't have a relationship outside the world of football, just as we don't with a lot of other players", before adding that in years to come he hopes they can laugh about it together, stating; "We have to look on this rivalry with a positive spirit, because it's a good thing."

Some commentators choose to analyse the differing physiques and playing styles of the two, while part of the debate revolves around the contrasting personalities of the two players: Ronaldo is sometimes depicted as an arrogant and theatrical showoff, while Messi is portrayed as a shy, humble character. When asked which he preferred, then FIFA president Sepp Blatter commented that Messi will "always get a lot of votes because he plays well and scores goals" before saying that Ronaldo "is like a commander on the field of play. This is the other side of football and it's good to have commanders on the field. They don’t have the same attitude and that gives life to football. One has more expenses for the hairdresser than the other but that doesn't matter. I can't say who is the best. I like both of them, but I prefer Messi."

At club level, with Messi and Ronaldo representing bitter rivals Barcelona and Real Madrid, the two players face each other at least twice every season in the world's most popular regular-season club game, El Clásico, which had a global audience of 400 million viewers in March 2014. Off the field, they are the face of two rival sportswear giants, Messi of Adidas and Ronaldo of Nike, despite the opposite kit sponsors for their respective clubs. The two highest paid players in football, Ronaldo and Messi are among the world's best paid sports stars: in combined income from salaries, bonuses and off-field earnings in 2015, Ronaldo was third earning $79.6 million, and Messi was fourth with $73.8 million. They have the two biggest social media followings in the world among sportspeople with a combined 195 million Facebook fans by April 2016.