Dimitrov, competing in the season-ending event for the first time, came out on top 7-5 4-6 6-3 in a time of two and a half hours in the English capital.
Victory for the Bulgarian, who is the first man in 19 years to win the tournament at the first attempt, is enough to take him up to third place in the men's singles rankings.
Both players struggled to hold serve in the early stages of the contest, with Goffin appearing to settle quickest as he twice broke his opponent in the first three games.
Dimitrov, unbeaten in five matches during the ATP Finals, battled back and took the opener with his fifth set point, but the only break of serve in the second set went in Goffin's favour to see the match levelled up at 1-1.
The 26-year-old then edged a tense opening game in the deciding set and successfully held for the remainder, taking the fifth match point when it arrived to eventually get over the line for the biggest title of his career.