The Belgium international believes the club must keep their "best player" in order to improve on last season's fifth-placed finish and qualify for the Champions League
After scoring 21 league goals last season, the Wales international has become one of the hottest properties in football, which has led to interest from Real Madrid.
Goal understands Bale is expected to sign a new contract worth €153,000 at White Hart Lane this summer and Dembele has urged the club to remain strong and resist any offers for their star man.
"He is an unbelievable player, so you never hope that your best player goes," he told Sky Sports. "That's my case as well.
"At the club we talk to each other a lot and he is a good guy."
With Bale at the club, Dembele believes Tottenham can achieve "big things" next season and the Belgium international is also hoping to improve in his second season with the north Londoners.
"We are looking forward to starting the competition again because I think we can do big things," Dembele continued.
"I always think I can do better. It was my first year [last season] and I hope next year I can do much better."