Nishikori beat second-seeded South African Kevin Anderson 6-4, 6-4 in the indoor hardcourt event.
Nishikori clinched his eighth ATP Tour title in what proved to be his easiest match of the week. He dropped the first set in each of his three previous matches before battling back to advance.
He needed consecutive tiebreakers to scrape through in a marathon semi-final against American Sam Querrey on Saturday, but took just 75 minutes to dust off Anderson.
"Today’s match was much different than (the past) couple of matches," the 25-year-old Nishikori told reporters.
"I played really solid from the baseline and returned really well."
Nishikori became the first Asian man to make a Grand Slam singles final when he finished runner-up at last year's U.S. Open.
Anderson has now lost six ATP finals since his last victory in 2012.
"Being in that stage a few times now in the finals, obviously you want to take it a little bit further, but it wasn’t there," he said.
"I have a couple of other tournaments coming up now, so I can’t afford to dwell on this."
Image: Kei Nishikori
Photograph: Hannah Peters/Getty Images
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