Barca did not publish details of the deal, which replaces the current contract that expired at the end of June 2018, but are expected to have enhanced Messi's status as one of the best-paid players in the game.
Local media reported Messi would be tied to the club a further year, to June 2019, and would be paid 20 million euros (16 million pounds) a season plus a possible five extra in performance-linked bonuses.
"FC Barcelona has reached an agreement to adjust the terms in the contract binding Leo Messi to the club as a professional first team player," Barca said in a brief statement.
"The revised and updated contract will be signed over the next few days," they added.
Argentine Messi is Barca's all-time top scorer with 354 goals in 424 official games and news of the deal is a boost for the Catalan club before Saturday's La Liga showdown against Atletico Madrid.
Since the 26-year-old made his official debut in the 2004-05 season, Barca have won 21 titles, including three Champions League triumphs and six La Liga crowns.
He joined Barca's academy at the age of 13 after a spell at Newell's Old Boys in his native Rosario.
(Reporting by Iain Rogers, editing by Justin Palmer)
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