The Argentina captain would also regain control of his image rights, one report said, further boosting the income of a man Forbes magazine estimates is the 10th highest-earning athlete, with annual revenues of $41.2 million (£24.5 million) including wages and endorsements.
"FC Barcelona announces that it has reached an agreement for the adjustment of the professional contract of Lionel Messi," a statement on Barca's website read. "The signing of this contract will follow in the coming days."
According to Eurosport's Spanish football expert Andy Mitten the contract is set to be worth €20m a year net which would make Messi by some distance the world's best paid player.
The news was announced on the eve of Barca's title decider against La Liga leaders Atletico Madrid, who travel to the Nou Camp on Saturday.
Barca need a win to take the title, with a draw enough for Atletico.
"Anything that is good for Messi is good for Barca," coach Gerardo Martino told a news conference on Friday. "It is a very good piece of news for both parties."
Since Messi made his debut in the 2004-05 season, Barca have won 21 titles, including three Champions Leagues and six La Liga crowns.
The four-time World Player of the Year is Barca's all-time leading scorer with 354 goals in 424 official games.
He joined Barca's academy at the age of 13 after a spell at Newell's Old Boys in his native Rosario.
Messi and his father last year paid 5 million euros (£4.1 million) to the Spanish authorities as a "corrective payment" after they were accused of filing false tax returns.
The pair, who both denied wrongdoing, allegedly hid more than 4 million euros by filing incomplete returns for the years 2006 to 2009.
The sale of Messi's image rights had been hidden using a complex web of shell companies in Uruguay, Belize, Switzerland and the United Kingdom, the prosecutor's office for tax crimes in Catalonia said.
- Sports & Recreation
- Lionel Messi
- Cristiano Ronaldo
- Real Madrid