Josh Taylor beat Regis Prograis on points to become the unified IBF and WBA super-lightweight champion and win the World Super Series in London.
The Scot, 28, earned a riveting, see-sawing victory, with judges scoring it 114-114, 115-113, 117-112.
Prograis was composed in the early skirmishes but Taylor grew into the contest and landed telling blows in the eighth, ninth and 10th, before two fiercely contested final rounds.
Taylor remains undefeated in 16 bouts.
He inflicted his American opponent's first loss in 25 fights en route to the Muhammad Ali Trophy, while also claiming the WBC Diamond and Ring Magazine belts.
With his right eye swollen shut, Taylor dedicated the O2 Arena victory to his late father-in-law, who died last month.
"He was here with us tonight and I want to dedicate this to him," he told Sky Sports.
"What a fight. All respect to Regis Prograis, a great fight, great champion, but the best man won.
"The free-flowing boxing, the inside work - I don't think he quite expected I could switch it up and go to range quite as quickly."
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'Danger everywhere in fight that hung in balance'
The fight was a mouth-watering prospect for boxing fans - two undefeated world champions, a spot of needle in the build-up and no fewer than five belts up for grabs in a packed stadium as the two southpaws prepared to battle.
Taylor, four years a pro, spoke of his speed, reactions and timing being his greatest assets: all were tested in a thriller.
He held the advantage in height and reach and was the man on the front foot as the Houston-based WBA champion relied on his head movement to evade Taylor's barrage before countering with both fists.
There was danger everywhere - and from both fighters. Crunching body shots, uppercuts that began as far south as Louisiana, head-popping jabs, fists flying at close range and big overhead lefts.
By the mid-point of the scheduled 12 rounds, there was little between the fighters but the momentum seemed to be moving in favour of the Tartan Tornado, two years younger than Prograis.
But the American, evoking memories of Terence Crawford coming to the UK to take home Ricky Burns' WBO lightweight title in 2014, remained lively if increasingly ragged.
Taylor's right eye was swollen by round eight as the two world-class talents traded blows before Taylor stepped up his work-rate in the ninth with shots to the body and head of his resourceful foe.
In the hardest fight of their respective careers, the result was in the balance. Another strong round for Taylor in the 10th would have demoralised a lesser opponent but not Prograis - he traded body blows, landed a cracking uppercut on the taller Scot and soaked up heavy shots.
Prograis was back to his cocky, stylish best with three minutes remaining, Taylor replying with his own form of artistry to rouse the crowd once more as the final bell sounded.
'Prograis let him into the fight' - analysis
Former middleweight world champion Andy Lee on BBC Radio 5 Live
"The one judge who gave the win by five rounds, I'm not sure what that was on about, but the other two scores were fair. Taylor found a new energy and I think Prograis let him into the fight."
BBC Radio 5 Live boxing pundit Steve Bunce
"What an outcome. What an hour it was to be there ringside. Prograis half made out he won the fight. The last two rounds it was a difficult 10-9 to put down."