The 32-year-old, the Black Cats' big-money January signing, thundered home the sweetest of strikes in first-half stoppage time.
He appeared tearful, affected by the emotion of the occasion, at the half-time whistle, and the goal proved all-important in a match Sunderland dominated, with the final whistle that confirmed the 1-0 win bringing relief and celebration
In the process, Defoe handed new head coach Dick Advocaat victory over the Magpies in his second game at the helm, remarkably a feat also achieved by both his immediate predecessors, Paolo Di Canio and Gus Poyet.
It was just Sunderland's third league victory on Wearside this season, and followed an eight-game winless run, but it was one they thoroughly deserved with Newcastle offering little in return as they slipped to a fourth defeat on the trot.
John Carver's men have been marooned on 35 points since they beat Hull on January 31, and while they should have enough in the bank to avoid being dragged into a survival fight, their season has unravelled alarmingly since Alan Pardew's departure.
The stakes could hardly have been higher for the home side, who had avoided dropping into the bottom three only by virtue of goal difference when Burnley emerged from their home clash with Tottenham with a 0-0 draw, and they started the game accordingly.
It was largely one-way traffic as a Newcastle team riven by injury and suspension found themselves camped deep inside their own half for extended periods with home goalkeeper Costel Pantilimon enjoying one of his quieter afternoons.
But despite their utter dominance, the Black Cats went into injury-time at the end of the first half with the score at 0-0 and Tim Krul having had only a handful of meaningful saves to make.
Advocaat chose to field three strikers once again with Defoe, Connor Wickham and Steven Fletcher deployed to turn the screw on a Magpies rearguard which included full-back Daryl Janmaat playing out of position at centre-half and midfielders Ryan Taylor and Sunderland old boy Jack Colback operating as full-backs.
Jonas Gutierrez, starting his first game for the visitors since August 2013 following his recovery from testicular cancer, came to Newcastle's rescue with just eight minutes gone when he cut out Defoe's cross to Jordi Gomez inside the penalty area after Fletcher played his strike partner in down the right.
Janmaat hacked the ball over his own crossbar after Sunderland defender Billy Jones had turned the resulting corner back across goal, and Jones was unfortunate to see a back-post header clip the post with the help of deflections off both Moussa Sissoko and Colback with 22 minutes gone.
Wickham forced a good save from Krul at his near post with five minutes of the half remaining, although the visitors gradually started to work their way into the game with Remy Cabella finally making an impact.
But Newcastle were dealt a devastating blow on the strike of half-time when Fletcher climbed high to head a long ball towards Defoe, who unleashed a stinging left-footed volley which flew past the helpless Dutchman and into the top corner.
Colback, to the delight of the home fans among a crowd of 47,563, dragged a left-footed shot across the face of goal early in the second half.
Newcastle were playing in a far brighter fashion as they looked to create problems for the home defence, but at the other end Fletcher spurned a glorious opportunity to extend Sunderland's lead when he scuffed weakly towards Krul from Gomez's inviting pass.
The visitors were at least making a fight of it, yet despite enjoying a much greater share of both possession and territory, they lacked the incision to do anything about the deficit.
Sunderland could have cemented victory twice within seconds as the match entered its final quarter, with full-back Patrick van Aanholt slicing wide and Fletcher lifting a shot wastefully over Krul's crossbar.
Sebastian Larsson curled a 75th-minute free-kick just wide, but Pantilimon was forced into his first real save by substitute Emmanuel Riviere three minutes later with the Magpies making a belated push.
Larsson might have sealed the win when his 84th-minute shot was deflected just wide by Mike Williamson, before Ayoze Perez had a golden late chance for Newcastle but smashed a volley just over with three minutes remaining.
Source : PA
Source: PA