The Catalan giants confirmed on their official website the Argentinian would pen the deal "in the coming weeks".
Messi's current deal runs until June 2016 and the new agreement will keep him at the Nou Camp beyond his 31st birthday.
The forward has enjoyed an extraordinary year, even by his own remarkable standards, netting his 89th and 90th goals of 2012 at the weekend.
The 25-year-old had already surpassed Gerd Muller's 40-year-old record of 85 goals in a calendar year.
Barca also revealed Xavi and Carles Puyol had agreed extensions until 2016.
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