0924 GMT February 18, 2020
Sixth seed Dimitrov, 26, claimed the biggest title of his career as he won 7-5, 4-6, 6-3 at London's O2 Arena, BBC reported.
The Bulgarian collected £1.9 million in prize money for coming through the tournament unbeaten in five matches across the group stage, semifinals and final.
He will end the year ranked third in the world, with Goffin up to seventh.
"I'm still trying to think about what I just did," said Dimitrov, who follows in the footsteps of Spain's Alex Corretja, who won the season-ending title on his debut in 1998.
"This is a great, unbelievable achievement for me, but I still have a lot to give.
"One of my main goals is to win a Grand Slam tournament. This has always been a dream of mine. Now, slowly, I think [I'm] getting there."
Injuries to Andy Murray and Novak Djokovic kept two of the sport's biggest names out of the ATP Finals, while world number one Rafael Nadal was forced to pull out injured after his opening round-robin match and Roger Federer was stunned in the semifinals by Goffin.
But Dimitrov said it was too soon to say a new era was coming to tennis.
"Next year I think is going to be obviously pretty interesting, especially the beginning of it," he said.
"There's still quite a few names that are going to come back and play. You should never count them out."
He added, "Am I one of those guys? Well, right now I'm right here. I'm the winner of the tournament. But that's about it.
"The important thing is just to stay on the ground and put your head down, even work harder because once you get to that point, everything becomes so narrow for you.
"I don't want to get too hyped up because I've done well, now I'm number three in the world. This makes me even more locked in, more excited about my work, and for what's to come."
While Dimitrov's season is now over, Goffin will try to help Belgium to a first ever Davis Cup victory against France next week.
"It was a special week for me," said the Belgian.
"A week with a lot of emotion and a lot of fatigue. Now I am feeling tired but it was an amazing week."