Swiatek, who has won four of the last five French Open titles at Roland-Garros, was reduced to tears in her post-match interview following the loss.
It ended the Pole's 1149-day unbeaten record on the clay courts in the French capital, having won 25 consecutive matches, along with a 47-4 completed sets record and claiming 10 sets by a score of 6-0 over that span.
The world number one was the pre-tournament favourite to win gold in Paris, having been knocked out at the quarter-final stage in Tokyo three years ago.
But a nightmare first set ended 6-2 in favour of Zheng, and when the Chinese broke Swiatek to draw level in the second after racing into a 4-0 lead, there seemed to be only one outcome.
“I just had a hole in my backhand. It happens rarely because it is usually my most solid strike,” Swiatek told Kuwik.
“I was not technically well positioned because of the stress and the fact that I played my games day by day. We didn’t have time to adjust that and work on that.
“I know that’s not the justification but I tried to correct that during the match. Today it didn’t work at all. So she used that to win the game.”
When asked why she slumped from 4-0 to 4-4 and ultimately 5-7 in the second set, Swiatek added: “It's the result of the match. So I just messed up.”
Swiatek still has the opportunity to take home a bronze medal when she faces the loser of the second semi-final between Croatian Donna Vekic and Anna Schmiedlova of Slovakia.
The former world number one fought hard, but in the end, could not hold off the sixth seed, who won 6-7 (4-7) 6-4 7-6 (8-6) at Roland-Garros.
The pair traded breaks in the first set, needing a tie-break to separate them before Kerber edged in front. And while the German matched her opponent for much of the second, Zheng won the final three games to force a decider.
It would take another tie-break to decide the winner after a battling third set, and though Kerber rallied after three consecutive match points, she lost her serve on the penultimate point, and Zheng took full advantage.
Zheng will face either world number one Iga Swiatek or Danielle Collins for the right to play for a medal.
Data Debrief: Kerber says goodbye
Kerber announced before the tournament that this was to be her last, and she pushed to the very end in what turned out to be her final game.
The 36-year-old won three grand slams in her career and has already made history in Paris by reaching the most women's singles quarter-finals at the Olympics (three) since 1988, equalling Arantxa Sanchez Vicario.
She is also one of only 15 players to stay 30+ weeks at the world number 1 spot since the WTA Rankings were first published in 1975 (34 weeks in total at #1).
China's Zheng, who ousted world number one and three-time reigning French Open champion Iga Swiatek at Roland-Garros en route to the final, prevailed 6-2 6-3 in Saturday's final.
It makes the 21-year-old the first Asian-born player to win gold in the women's or men's singles at the Games since tennis was first introduced to the Olympics in 1896.
Zheng started the encounter fiercely, earning an early break in the second game and raced into a 3-0 lead on Court PhilippeChatrier.
The world number seven would stay in cruise control for the remainder of the opener, seeing out her service games before ending the set with another break.
Zheng threatened to take the match away from Vekic in double quick time as she again found a swift break of serve in the second set.
Croatia's Vekic responded well, though, hitting back by reeling off back-to-back games.
However, a break at the fourth time of asking to make it 5-3 proved decisive for Zheng, who duly served out to clinch gold.
Data Debrief:
Zheng's triumph saw her become the second player to win the women's singles at the Olympics on clay after Jennifer Capriati (1992), since the sport's reintroduction to the Games in 1988.
She has now won 28 matches out of 36 (77.77) on the surface since the start of 23. Among players with 10+ matches played on clay in that time, only three have a better win percentage - Swiatek (91.7%), Elena Rybakina (85.2%) and Aryna Sabalenka (80.6%).
Zheng is the first Chinese player in the Open Era to claim a medal in the women's or men's singles at the Olympics, while she also joined an elite list of women to have reached the finals of the Olympics and the Australian Open in the same season.
Steffi Graf (1988), Justine Henin (2004), Maria Sharapova (2012) and Angelique Kerber (2016) are the other players to have achieved that feat.
The sixth seed took one hour and 50 minutes to get a 6-2 7-5 win on Court Philippe Chatrier, earning her first-ever win over Swiatek.
Zheng's last two matches have both lasted three hours, but there was no sign of tiredness as she quickly stated her intent.
An early break each had the players level at 2-2, but Swiatek lost her next two serves as well, with Zheng taking advantage of her uncharacteristic mistakes to take the first set after a four-game winning run.
The Pole looked back to her usual best at the start of the second, racing into a 4-0 lead before stumbling again as Zheng's aggressive approach saw her claw back to level 4-4.
The reigning French Open champion then had no answers after edging in front again, losing her serve once more as Zheng won the final three games to book her place in the Olympics final.
She will play either Croatia's Donna Vekic or Slovakia's Anna Karolina Schmiedlova for the gold medal, while Swiatek could still take bronze.
Data Debrief: Zheng ends Swiatek's Roland-Garros run
Swiatek was the favourite to take the gold medal at the Olympics, especially considering her 20-match winning run on clay heading into the semi-finals.
Not only that, but she was unbeaten at Roland-Garros for 1,149 days, winning 25 consecutive matches, along with a 47-4 completed sets record during that span.
But, the five-time grand slam winner had no answers for Zheng on Thursday, who prevailed for the first time in seven meetings against Swiatek.
