Golovkin improved to 27-0 with his 24th knockout and 14th in a row, using a left hand to the body to flatten Macklin (29-5) 1:22 into the third. Golovkin dominated the first two rounds, landing several hard combinations.
"It was an easy fight for me," Golovkin said. "I felt great in the ring. He never hurt me. I was very happy with my performance."
Golovkin has made five successful title defenses and hasn't gone the distance in a fight since June 2008 against Amar Amari in an eight-round bout.
Macklin has lost three of his last four fights. He lost a split decision to Felix Sturm in June 2011 for the WBA super middleweight crown, and dropped another title shot in March 2012 to Sergio Martinez.
"He never really let me get started," Macklin said. "It was a great shot he landed. I tip my hat to him. He's a great champion. He has plenty of clubbing, solid power, and you can feel the weight of every punch he throws."
There was talk after the bout of a potential fight between Golovkin and Martinez, the WBC middleweight champion. Martinez, however, is nursing knee and hand injuries, and isn't expected to resume boxing until 2014.
"I want to fight again very soon," Golovkin said. "I want to fight any belt-holder, any champion, and any top fighter, anywhere."