Eddie Howe’s strugglers looked to be heading for a fifth straight Premier League defeat when Solly March headed Brighton in front 10 minutes into the second half.
But strikes from Andrew Surman and Defoe, who made his name at Dean Court as a teenager on loan from West Ham, at last gave boss Howe reason to smile.
The Cherries suffered a pre-match blow when Steve Cook was ruled out by knee injury, denying the defensive stalwart the chance to line up against his old club.
They dominated possession from the kick-off but it was Brighton who mustered the first effort on goal as Anthony Knockaert drilled a low shot wide from 22 yards.
Bournemouth midfielder Harry Arter responded by ballooning a long-range effort over the crossbar before defender Shane Duffy almost gifted the hosts the lead in the 15th minute.
Duffy miscued a clearance straight to the feet of Joshua King but Lewis Dunk came to his teammate’s rescue with a heroic block on the edge of the penalty area.
Brighton were carved open moments later when Charlie Daniels outstripped Liam Rosenior before teeing up Marc Pugh, who could not keep his effort down.
Daniels, whose blockbuster against Manchester City was Bournemouth’s only Premier League goal coming into the game, then hammered a shot down the throat of Brighton goalkeeper Maty Ryan.
Brighton showed little attacking intent in the opening half an hour but Davy Propper came close to a spectacular opener when he chipped over the bar after a sloppy Arter pass.
Defoe was only a few inches off target with a swivelling shot from outside the box, which had Ryan scrambling across his line.
Brighton came out guns blazing after half-time and would have been in front but for a brilliant double goal-line clearance from Surman.
First Surman kept out Shane Duffy’s header from point-blank range before reacting superbly to block Propper’s follow-up effort.
Bournemouth had another narrow escape from the resulting corner when Dale Stephens rattled the crossbar with a header.
But Brighton did not have to wait long to delight the Seagulls fans who had made the 100-mile trip along the south coast.
Pascal Gross, two-goal hero in last weekend’s maiden Premier League win over West Brom, sent in a superb cross from the right and March rose highest to nod past a helpless Asmir Begovic.
Bournemouth chief Howe sent on £15m former Liverpool winger Jordon Ibe from the bench in a desperate bid to rescue the game.
Ibe made an immediate impact, playing a neat one-two with Surman before the former Southampton midfielder jinked his way into space in the 18-yard box and side-footed home.
Super sub Ibe had a hand in Defoe’s winner six minutes later, picking out the England striker for a delightful first time finish after Brighton had only half cleared a corner.
Defoe’s last goal in Bournemouth colours was during a 3-3 draw with Reading in the old Division Two in May 2001.
