The 29-year-old touched in 1 hour 59min 30.8sec - only 0.9 seconds in front of defending champion Sharon van Rouwendaal of The Netherlands in 1:59:31.7.

Fast finishing Australian Kareena Lee took bronze in 1:59:32.5.

Cunha, who dyed her hair in the Brazilian national colours of yellow and green, took the lead on the first lap and was never out of the top four after that.

She is the first Brazilian woman to win a swimming gold, which rectifies an omission in her illustrious career.

After making her debut at Beijing 2008 as a teenager, she also competed at her home Rio 2016 Olympics but a medal had always eluded her until today.

"We arrived here wanting this medal as much as you can," Cunha said. 

"Around 10 days ago, I said to my coach, to win this race will be very difficult for my opponents, because I want it so hard, so much and I'm really well prepared.

"We had to be cool or 'cold' as Europeans are. 

"We are Latin people, we are hot, we are emotional people, so I had to be very cold mentally in the race to be focused and I had to win it myself. 

"I knew I was prepared for that."

Rio 2016 gold medallist Van Rouwendaal is the first woman to win two Olympic medals in marathon swimming.

"The silver feels like gold," the Dutch racer, who moved to Germany and changed coaches, said.

"Today I felt like one of the favourites and that people would watch me, so I had to come with a strong plan.

"This was my best performance in warm water.

"At the end I was hiding in fourth or fifth place, I didn't want to take the lead today because people would have overtaken me at the end.

"So I think this was the best I could have done."

Lee is the first Australian to win a marathon swimming medal.

She missed out at Rio 2016 and revealed that COVID rules had restricted much of her training programme to the pool.

"I like to conserve energy for the last two laps, the race ended up kicking up probably from the second lap which was a little bit unexpected so I dropped back a bit," she said.

"But I was still in a comfortable position for me.

"The plan was I'd start to pick up the pace and that was when Ana started to go as well, so I made the decision to follow her.

"Because of the COVID situation I'd been doing a lot more pool work and I think the speed really paid off.

"Mentally staying there at the end was really important."

The race began here at 6.30am in a bid to beat the worst of the Tokyo heat.

The men's race is tomorrow.