While the argentinian superstar agreed to a contract extension until 2021 in July, Messi is yet to put pen to paper on the contract with the Catalan club.
It has raised doubts over Messi’s future at Camp Nou following Neymar’s world-record transfer to Paris Saint-Germain.
Barca have continually reiterated that the 30-year-old will sign his contract and Bartomeu provided another update on Thursday.
“We will take the photo with Messi’s signature, hopefully before the end of the year, but the contract has been signed by his father, who has the power of his image rights,” Bartomeu told 8TV.
“This contract is signed until 2021 and he is already playing under it.”
Barcelona beat Juventus comfortably 3-0 on tuesday night in the first game of the Champions League group stages.