Barca announced the Argentina superstar was to extend his stay at the Nou Camp by a further two years in December. When he puts pen to paper he will be tied to the Catalan giants until June 30, 2018.
Messi is currently with the Argentina squad ahead of Wednesday's friendly in Sweden but will sign at the club offices on his return a day later.
He becomes the third big-name Barca player to commit his future to the club, following in the footsteps of Carles Puyol and Xavi.
Messi won his fourth straight FIFA World Player of the Year award in January following a remarkable 2012 in which he scored 91 goals - smashing the world record of 85 in a calendar year set by Bayern Munich and West Germany striker Gerd Muller in 1972.
His goal in Sunday's 1-1 draw at Valencia was a league-record 12th consecutive game in which he found the net.
Overall, he has scored 203 La Liga goals for Barca in 236 games.