The British number one, who won in Brisbane last year when he also claimed Olympic gold and the US Open title, won 7-6 (7/0) 6-4.
The Australian Open kicks off on Monday January 14.
Having beaten Dimitrov the only other time they had met in Bangkok in 2011, Murray could have been forgiven for being confident but he did not have it all his own way against an opponent ranked 48 in the world.
First-time finalist Dimitrov got off to a storming start, winning 11 of the first 12 points and breaking Murray in game two to go 3-0 up.
But Murray broke back as Dimitrov served for the set at 5-3, then forced a tie-break which he took to love.
Dimitrov blasted a forehand winner down the line to hold serve for 2-2 as the second set went with serve.
A deciding set looked possible when the Bulgarian broke to lead 4-3 but Murray took the next two games to go 5-4 ahead.
With the set back on serve, Murray broke once more to retain the title.
Murray, in quotes carried on the tournament's official Twitter feed, said: "To everyone who put on the event, thank you."
Dimitrov, playing in his first ATP final, said: "It was a good experience. Hopefully I'm going to come back next year and do better."