With four days to go until International Olympic Committee (IOC) members meeting in Buenos Aires choose between the Spanish capital, Istanbul and Tokyo, Messi said it would be positive to award Madrid the Games given the nation's recent success in a number of high-profile sports.
"I don't know whether my support will help," the World Player of the Year, wearing a shirt with the Madrid logo, said in a video published by Spanish bid officials.
"I think it is very important for a city to host the Games," added the 26-year-old, who has lived in the Catalan capital for more than a decade.
"I was fortunate enough to participate in the Olympics and it is a fantastic experience for any athlete and I think it could be something very important for the city of Madrid.
"I think Madrid has a good chance for what it (the city) represents, Spain's position in the world of sport, where it has triumphed in many different areas, and that can have an influence too.
"I think it would be good for sport to award Madrid the Games and I support their bid from here."
Messi is also a brand ambassador for Turkish Airlines, one of the main backers of Istanbul's bid, and features in advertisements currently on display in Buenos Aires promoting the airline's services to Latin America.
Some reports suggested he had opted not to promote the Madrid Games candidacy because of his involvement with the airline but this was dismissed by Spanish Olympic Committee (COE) president Alejandro Blanco on Tuesday.
"He (Messi) never declined to support the Madrid bid," Blanco said from Buenos Aires, where he met with Messi at the Argentina football federation (AFA) headquarters.
"There was a timetable issue because he was training," Blanco added.
"It is a great honour for us to have his support, for what he represents as a sportsman.
"And I think more importantly for what he represents as a person for the entire world."
Madrid's bid also has the support of the Los Angeles Lakers' Spanish forward Pau Gasol.
(Reporting by Iain Rogers, editing by Toby Davis)
- Sports & Recreation
- Lionel Messi
- Buenos Aires
- 2020 Olympic Games