The 29-year-old cemented her status as the greatest all-round women's track and field athlete of her generation with a third consecutive gold in the gruelling multi-discipline test of endurance.
Thiam, the 2016 and 2020 Olympic champion, finished with 6,880 points after the 800m, pipping Britain's Katarina Johnson-Thompson took silver with 6,844 points.
Thiam's compatriot Noor Vidts won bronze with 6,707 points.
"I didn't cross that line and think that I made history," said Thiam, the first woman to win three heptathlon golds.
"I thought all this pain, hard work, sacrifice, all those moments where I felt lonely. All that pain, all that hard work, all of that paid off and I'm really grateful for that.
"In sports you give everything you have, you need to, in every moment. But you have to take whatever the sport gives you, and it can be nothing. Today it's a lot, and I'm grateful for that."
A year ago Thiam faced an uncertain future after withdrawing from the World Championships with an Achilles tendon injury -- a potentially career-threatening issue for heptathletes given the demands of the event.
"It was not easy way to get where I am," Thiam said. "It's easy to support me today, and there were a lot of days where I needed the support when it was actually hard.
"To the people who were there in those moments, had the kind words, just a hug. I thank them and dedicate this to them."
Thiam had put one hand on the gold after Friday's morning events, when a massive 54.04m javelin throw left her 121 points clear of Johnson-Thompson after six events.
That left Thiam needing to finish within eight seconds of Johnson-Thompson's time in the 800m.
Johnson-Thompson battled bravely down the stretch in the 800m to put as much distance between her and Thiam, finishing second in 2min 04.90sec.
But a weary Thiam just managed to scrape inside the eight-second window, crossing in 2:10.62 to claim her third Olympic gold.
Thiam was coy about whether she might be around in 2028 to go for a possible fourth gold.
"It's always about the future, more, and more and more," she said. "What I want is to enjoy it, and nobody can take that from me.
"I'm here now, I need to take this now. I worked too hard, I need to enjoy this moment now."
