Triple Player of the Year Bale scored a career-best 19 Premier League goals last season, but Spurs were still edged out by arch-rivals Arsenal in the race for a top-four finish.
Spanish giants Real Madrid are thought to be willing to test Tottenham's resolve to keep Bale by tempting them with a £60m offer this summer.
However, Daniel Levy is prepared to reject all bids for the 23-year-old and Defoe has backed his chairman's stance, suggesting that Spurs cannot move forward if they allow their best players to leave.
"It is important to keep your best players and it doesn't make any sense selling your best players," Defoe told Sky Sports News.
"Gaz had a fantastic year last year and the Premier League is one of the best leagues in the world. What he did last year was outstanding and if you do that you're going to have other clubs after you.
"For me, I want us to keep all of our best players as I want to play with top players. We want to get in the Champions League and we need to keep our top players."
The Lillywhites gave a strong indication that Bale will remain at White Hart Lane by having him model their new kit for the 2013-2014 campaign.