Messi's future has been the subject of intense speculation this week, with rumours suggesting that Ligue 1 leaders Paris Saint-Germain are considering a bid for the Argentina attacker, whose contract at the Camp Nou expires in June 2018.
The reports come a month after club vice-president Javier Faus angered the 26-year-old by suggesting that there was "no reason" to review his deal with the Catalan outfit.
Bartomeu took over as president after Sandro Rosell resigned last Thursday amid reports that the Spanish courts are to open investigations into alleged misappropriation of funds surrounding the deal to bring Neymar from Santos to Barca.
And the 50-year-old revealed that he is keen to ensure that Messi is the best paid player in world football.
"He (Messi) is not for sale," Bartomeu told RAC1.
"We want to sit down, without any rush, with Leo's father (to negotiate a new deal).
"It's not something that's happening tomorrow, we have plenty of time, but we will do what we have to ensure he's the best paid player."
Bartomeu also admitted that he attempted to persuade Rosell to stay at the club, despite the furore surrounding the Neymar transfer.
"Along with everyone else, I spent the whole day trying to persuade him (Rosell) but it was impossible," he explained to beIN Sport.
"We believe that he did an excellent job during the years that he was in charge at Barcelona. Now I can only promise hard work, effort and dedication so that Barcelona continues growing as a club every day."
