The Australian world number one won 6-4 6-3 to take the season-ending title in Shenzhen and a record $4.42m (£3.42m).
Barty, 23, had lost all her five of her previous matches against the Ukrainian defending champion.
But she came through a testing second set, featuring five breaks of serve, to take victory when Svitolina netted.
Barty, who won her first Grand Slam title at the French Open in June, will end the season as the world number one, while 25-year-old Svitolina, who reached both the Wimbledon and US Open semi-finals, finishes a year without a title for the first time since 2013.
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Barty ends stunning year with another major achievement
Whatever happened in the Shenzhen final, Barty’s lengthy list of achievements in a spectacular season would not have been diminished.
But she made sure she capped an extraordinary year by becoming the first Australian to win the WTA Tour finals since her role model Evonne Goolagong Cawley in 1976.
What makes her success even more remarkable is the fact Barty stepped away from the tour following the 2014 US Open, saying she felt tennis was a “lonely sport”.
The Queenslander switched to cricket and played for the Brisbane Heat in the women’s Big Bash before returning to the WTA Tour in early 2016.
After rising to the top 20 last year, Barty’s unique mix of power and variety, intelligence and creativity, has taken her to the top of the women’s game with titles on all surfaces this season.
Her 2019 achievements include:
- Winning first Premier Mandatory title at the Miami Open
- Claiming her first Grand Slam singles title at the French Open
- Winning the Birmingham title to rise to world number one for the first time
- Finishing the year as the world’s top player
- Becoming the fifth player to win the WTA Finals on her debut
“It has been a remarkable year tonight was about coming out and fighting to the end. I couldn’t be prouder. It has been an incredible year,” Barty said.
Event finishes on high with engaging final
An engaging final between the top seed and defending champion was a positive end to a season-ending finals which did encounter some problems on its debut in Shenzhen.
The slow speed of the courts was criticised, while crowds in the early part of the tournament were not as packed as organisers would have hoped.
But the major issue threatening the credibility of the tournament – and the tour itself – was the gruelling demands on the players over the course of the season resulting in four of the final eight withdrawing through injury.
Japan’s Australian Open champion Naomi Osaka was the first player to pull out because of a shoulder injury, before US Open winner Bianca Andreescu also withdrew as her fitness issues continued with a knee problem.
Dutch alternate Kiki Bertens, Osaka’s replacement, could not continue because of a viral illness and the lengthy list was rounded off by Swiss seventh seed Belinda Bencic quitting her semi-final against Svitolina with cramp.