According to Spanish journalist Guillem Balague, the new deal will surely be signed, as all the terms have been agreed.
Messi has not signed his new contract with FCB. But it is all agreed and WILL sign it pic.twitter.com/Z4Q7C1cZJY
— Guillem Balague (@GuillemBalague) August 20, 2017
The details of the new contract are not yet revealed, however, it has been previously reported that the new deal will be in force till summer 2021. His current contract expires in June 2018.