It was Golovkin's 17th world middleweight title defence and reaffirmed the Kazakh's status as the No. 1 in the division His latest victory came after British challenger Brook's trainer Dominic Ingle threw the towel in the ring to prompt referee Marlon Wright to stop the fight.
Even though Brook was the smaller man, they appeared to be the same size, but as the fight wore on it was clear Golovkin's shot were far more damaging.
The next blow though was a typical left to the ribs that left Brook open for a simple right hook in to the head that had him in trouble. Gennady carries bad power in both hands and he cranked up a remorseless intensity.
Golovkin's accurate punches began to trouble the Briton from the third round. Gradually it became harder and harder for Kell to keep him off.
Brook was always going to be up against it moving up from welterweight (where he is IBF champion) to the full middleweight limit to take on such a feared puncher.
And the champ showed why he's one of the best around as his onslaught forced Brook's corner to throw in the towel.
Overall, it was a very good fight - one to watch for sure.
The Sheffield boxer revealed post-fight that he thought he had broken his eye socket in the second round, with that injury prompting the surrender from coach Dominic Ingle and the subsequent visit to hospital.
"I was starting to come on. I would fight him again, with a good eye". When you see five of them against a great champion, it's tough. It earned him plenty of boos from a sold-out crowd at the O2 Arena, but the champion knew Brook was hurt.
Gennady Golovkin beats Kell Brook in fifth round to retain middleweight titles
"We'll go back to the drawing board, go on the machines". I wish him the best of luck but I don't see him going past six rounds. The idea of a fight with Canelo Alvarez is still on hold until September 2017. I've jumped up two weight divisions and boxed a man everyone is running away from. My eye stopped me coming forward.
"Gennady, you side-stepped me once".
"I was getting caught, and getting caught".
Fight fans can watch the Golovkin vs. Brook championship fight via HBO NOW.
BILLY JOE SAUNDERS has challenged middleweight king Gennady Golovkin to fight him in a winner-takes-all fight.
"I don't feel power". I expected him to be a bigger puncher. "I think Kell deserves lot of respect".
"Thank you very much London".
He added: "I am a warrior, I want to carry it on, but my team are watching it from the outside". With the win, Golovkin continues his unbeaten streak of 36 wins since turning pro in 2006 while Brook dropped to 36=1. I want to shock the world.
"Whatever date Golovkin wants to do, I will be ready".