Chris Hughton's charges took the lead just after the break through Solly March but the introduction of Jordon Ibe in the 65th minute changed the game for the hosts as he notched up two assists to set up Andrew Surman and Defoe to score.
The result ensures that Eddie Howe's side now have their first Premier League points of the season, keeping them in 19th place in the table but level on points with West Ham United and Leicester City.
The hosts, however, soon took hold of the game and applied pressure to their opponents, with Charlie Daniels finding Marc Pugh inside the Gulls area with a first-time pass in the 20th minute, only for Pugh to miscue his shot and blaze wide.
On 24 minutes, Bournemouth weighed in with the first shot on target of the encounter as a Brighton attack was snuffed out when March was caught in possession and Daniels broke quickly, but he decided to have a go from range and Mat Ryan was able to make a comfortable save.
The best chance of what turned out to be a fairly tame first half came on 30 minutes as Defoe turned and let rip from just outside the area, only for the ball to sail just past the post.
Brighton were invigorated after the restart as Shane Duffy and then Davy Propper's headers from a corner were cleared off the line by Surman on 50 minutes.
The breakthrough came 55 minutes in as Gross beat his marker on the right before crossing into the box for March, who made the most of lacklustre defending to head home.
However, Ibe came on for the Cherries in the 65th minutes and, just two minutes later, backheeled the ball into the path of Surman, who evaded one challenged and slotted the ball in beyond the far post.
The former Liverpool man then came close to getting on the scoresheet himself on 71 minutes, only for Lewis Dunk to make a last-ditch tackle and deflect his goalbound effort over the bar.
It mattered little, however, because three minutes later, Ibe cut open the Gulls' defence with a through-ball to Defoe, who fired under Ryan to claim his first of the season, his first for the Cherries in 5,977 days, and what turned out to be his side's winner.