Emelianenko (37-5) finished Sonnen (30-16-1) via TKO at 4:46 of the opening round at Bellator 208 at Nassau Coliseum. The finish came when Sonnen curled up in the fetal position on the floor, as Emelianenko dropped hammerfists from the top.
The victory sets up a vacant heavyweight title fight between Emelianenko and Ryan Bader (26-5), who defeated Matt Mitrione by unanimous decision at Bellator 207 on Friday. The title fight will take place on Jan. 26 in Inglewood, California.
Bader is also the promotion's light heavyweight champion. That belt will not be on the line when the two meet.
Sonnen, of West Linn, Oregon, has fought the majority of his career at middleweight but moved up to be a part of Bellator's Heavyweight Grand Prix. The size difference was notable throughout the fight, including the opening seconds, when Emelianenko dropped him with the first left hook he threw.
Sonnen, 41, quickly bounced back to his feet and spent the majority of the bout trying to wrestle Emelianenko to the ground. He had some success doing so, ducking under a winging shot at one point to plant Emelianenko on his back, but Emelianenko's large frame proved difficult to control.
Emelianenko never appeared in any duress, though he did look a little tired after several grappling exchanges. Nevertheless, he improved to 7-1 in his past eight bouts. He retired from 2013 to mid-2015 but has looked good since returning to the sport. He knocked out former UFC champion Frank Mir in the tournament quarterfinals in April.
Sonnen, who advanced to the semifinals by defeating Quinton "Rampage" Jackson in January, suffered his first loss since January 2017.
Henderson decisions Awad, reminds fighters to conduct themselves better
"Whatever you do in life, it's about how you conduct yourself," Henderson said. "Your sons and daughters are going to watch how you act. How are you going to explain, 'I jumped off the cage because this guy talked bad about me. I was selling a fight.' How are you going to explain that?
"It's about how you conduct yourself as a professional athlete."
Henderson, who trains out of Glendale, Arizona, delivered that message following arguably the best performance of his Bellator career. He took Awad down in all three rounds and threatened to submit him with arm triangle chokes in the second and third rounds. On the feet, he landed clean kicks to the body.
All three judges scored it a dominant performance by Henderson: 30-26, 30-26 and 30-27. Prior to the fight, Henderson said it was the last bout on his Bellator contract, which he signed in 2016. He has gone 3-3 in Bellator.