The former world number four and current Olympic champion was a wildcard entry in Beijing, having struggled with form and fitness after elbow surgery in 2025.
It was the 54th-ranked Zheng's first title in nearly two years, and tears filled her eyes as she fell to the court in celebration.
"The feeling was incredible to win here in Beijing... in front of my home crowd is one of my dreams," she said during the trophy ceremony.
"I've been through a very, very tough year," she added.
She congratulated Andreeva on her French Open win from earlier this year before serenading the crowd with a song about chasing the "light of dreams."
Zheng, 24, is the first Chinese player to win the prestigious tournament since its inception in 2004.
Andreeva, ranked five, was more consistent to start with, but Zheng grabbed back a break and stayed with her 19-year-old opponent as they battled on the baseline.
Zheng saved three set points to level at 5-5 and won every single point after that, sending down an ace to take the first set.
A crowd desperate to see their homegrown talent crowned champion was at times disruptive as cheers erupted before points ended and shouts for Zheng punctuated serves on both sides of the net.
But the players kept their focus, holding serve until Zheng found a way through in the ninth game of the second set, firing brilliantly against the line to break and go up 5-4.
A double fault threatened to derail an imminent triumph until Zheng, defiant, sent down an ace to win on her first match point.
It is Zheng's first trophy since October 2024 when she beat Sofia Kenin in Tokyo, and her first-ever title at the WTA 1000 level.
The players' only previous meeting was in the 2024 China Open quarter-finals, where Zheng beat Andreeva in three sets.
Andreeva, playing in her first China Open final, said it was "a very nice week" despite falling just short.
"I love playing in Beijing, so of course I had a very, very nice time."
With her run to the final, Andreeva is set to move up to three in the world rankings for a career-high.