Judges scored the bout 114-112, 115-112, 117-109 – all in the 29-year-old’s favour.
The result will have been a shock to DeGale, 33, who even confidently suggested the loser should retire.
Eubank Jr will be pleased to have settled the long-standing grudge at the 02 Arena following a war of words that has lasted for years.
Following his win, Eubank said: “I’m back where I need to be – at the top of the food chain and now I’m coming for all the other belts in the super-middleweight division.
“I knew he (DeGale) would come in here and run, I’ve been working on my jabs, he is a very slick southpaw but the gameplay worked, smart pressure, not getting too ahead of myself, picking my shots and choosing my time to attack.”
DeGale praised his opponent and added he “didn’t do enough” to see off Eubank Jr.
He continued: “I’ve left my mark on boxing. Chris is tough, he is a good prospect, he was on it.
“I just didn’t do enough there are a lot of wild punches, punches I didn’t see. He was nicking the rounds and I have to go back and watch it, I just didn’t do enough.”
“Now I’m coming for all the other belts in the super-middleweight division”
Chris Eubank Jr
Eubank Jr’s first major break came when he cracked DeGale with a solid right hook – stunning the Hammersmith-born brute. DeGale was able to collect himself, however, and carried on the fight.
The fight picked up again for Eubank Jr in the 10th when he landed a strong jab, outclassing DeGale before knocking him down onto the canvas.
DeGale, however, managed to make it through to the next round.
By round 11 DeGale appeared beat, his posture unsteady, while Eubank Jr dominated and raring to finish the former Olympic gold medalist.
Eubank Jr’s domination continued into the 12th, trying to land a finishing blow while DeGale couldn’t respond.