Talks between Messi and Barcelona have been ongoing for the past year. Barcelona CEO Oscar Grau even suggested last month that the club will offer the Argentine a lifetime contract similar to the one Andres Iniesta signed.
There's been no confirmation yet from the Catalan side, but Tebas is confident that the 30-year-old has already signed a deal that will keep him in Barcelona beyond 2018, when his current contract expires.
"Messi has a contract which has been renewed," Tebas told reporters, according to Spanish outlet Marca.
"Contracts are official when they are signed and not when it is announced that they have been signed. Messi has signed with Barcelona; if I have not been lied to, he has signed."
Messi has previously suggested he would eventually prefer to return to Argentina to finish his career with boyhood club Newell's Old Boys.
Doubt over his future in Barcelona was initially fuelled when Messi reportedly told club officials last year he would prefer to wait until 2018 before considering his future.
However, according to Tebas' claim, the five-time Ballon d'Or winner may have already committed to staying at the club.