The four-time Ballon d'Or winner's deal expires in 2016. However he will sign a contract that will keep him at the club until for a further two years, according to Goal.com.
Messi is expected to put pen to paper on Thursday as he becomes the third influential player to sign for the club in 2013 after captain Carles Puyol and vice-captain Xavi furthered their stays at the club earlier this year.
It is understoodthat the Argentina international will earn around £9.5m per year after taxes and reportedly has a buy-out clause of £216m inserted into the contract.
Messi has made over 350 appearances for the Catalan giants, scoring 300 goals in the process.