From 3-0, 15-0 down in the opening set, Goffin won 15 consecutive points to take control of the match, and never relented.
But as time ran out for Dimitrov, Goffin played a poor service game, going long on a forehand to concede another break of serve as the Bulgarian levelled the match at 4-4.
Goffin saved three match points to ask Dimitrov the serve it out, but he kept his composure to seal a famous win on his fifth match point, although he was given a helping hand as the Belgian dumped a simple-looking volley into the net.
Goffin recovered from a set down to stun favourite Roger Federer on Saturday night, but he was second best in Sunday's final. The Swiss on Saturday was shocked by Belgian tennis player Goffin when he came back into the match to turn it in his favour and win 2-6, 6-3, 6-4 at the O2 Arena in London.
Yet the Swiss and Dimitrov will take home a record prize of $2.5m should they win the ATP Tour's season-ending event as undefeated champions.
Federer, the World No. 2, snaffled the opener in 33 one-sided minutes as he feasted on Goffin's steady medium-paced game.
"It has been one of the best weeks I have ever had", he said.
Goffin finally got himself onto the scoreboard after more than 10 minutes of play, forcing errors off Federer's solid baseline game with some precise and accurate serves.
The Belgian opened the match well, acing Federer twice, but he threw away a game point allowing his opponent to swoop in and claim the first game easily.
He was broken again in the third game of the second set and then twice more as Goffin cruised to victory.
Goffin, 26, landed just 42 percent of first serves in the first set and made 20 unforced errors, struggling to find the bite and consistency he had enjoyed in beating Federer. Earlier in the week, Goffin also beat world number one Rafael Nadal in three sets in the round robin, although the Spaniard appeared to be bothered by a knee injury and withdrew from the tournament after the match.
"But I was very happy the way I handled the pressure".
In spite of the overwhelming score line, Carreno Busta didn't actually play badly, but instead, all credit to Dimitrov who displayed a near-perfect performance.
Dimitrov held confidently and then broke when Sock mishit a rally ball.
With the pendulum suddenly swinging, Goffin took advantage of two double faults from the racquet of Dimitrov, smashing a forehand victor past him to seal the break in the next game. "I think that was the difference today".