Humbert saw off second seed Dimitrov 6-4, 6-3 with a solid serve and an impressive baseline game in the southern port city.
"I'm very strong mentally in finals," said the fourth-seeded Humbert, who has now won all five finals he has played on the circuit.
"Everything is there. I am pretty solid from the baseline, I serve very well and I am strong mentally," said Humbert, who also won in his home city of Metz last November.
Dimitrov, a former world number three now ranked 13, showed signs of fatigue after Saturday's three-hour semi-final battle with Russian Karen Khachanov.
Humbert broke in the 10th game to snatch the opening set. He saved break points early in the second set, racing to a 3-0 lead and keeping Dimitrov at bay with 87 per cent of points won behind first serves.
The Frenchman had lost to Dimitrov in both previous meetings.
On Monday, he rises to a new career-high of No. 18 in the ATP rankings to become the new French number one.
"It's a good feeling to win Marseille. It was here where everything began, where I reached my first semi-final on Tour," he added.
