The Argentina playmaker, whose contract was set to run out in 2014, becomes the best paid player at the club while his release clause has also been increased from 150million euros to 250million.
The 22-year-old joined Barca as a youth-team player in 2001 and has won a host of trophies with the Catalan giants, including last season's unprecedented treble.
"This Friday Leo Messi extended and improved his contract which links him to FC Barcelona," read a statement on www.fcbarcelona.cat.
"The Argentinian will be azulgrana until 2016 and has, from now, a release clause of 250million euros (£153million).
"The signing of the contract took place this Friday afternoon in the club offices.
"Apart from the player and president Joan Laporta, also present were Leo Messi's brother Rodrigo, vice-presidents Rafael Yuste and Joan Boix, technical secretary Txiki Begiristain, and football director Raul Sanllehi.
"With this new contract, Leo Messi ensures he will be azulgrana for 15 years, having started his career at Barca in 2001, in the youth sectors of the club."
The announcement had been expected and earlier coach Pep Guardiola expressed his delight at the news.
"It's an immense piece of news for the club and for the player that Messi is renewing his contract," said Guardiola, "as it also was when Victor Valdes and Xavi did so. People go to football to see the best and that's what they are."